Updates version branding from 13.0 to 13.1 across templates, automation, and test fixtures. Template directory lifecycle (creating 13.1 content folders, locking 13.0) is intentionally deferred.

Changes

Version & Automation

• eng/Versions.props: MinorVersion 0 → 1 (package versions now 13.1.0-*)
• .github/policies/milestoneAssignment.prClosed.yml: main branch milestone 13.0 → 13.1

Template Branding (8 templates)

• Updated AspireVersion choice, displayName, description, defaultValue: 13.0 → 13.1
• Updated conditions: AspireVersionActual == 13.0 → == 13.1
• Updated isEnabled predicates: AspireVersion == 13.0 → == 13.1
• aspire-empty: aspire-13.0-tfms → aspire-13.1-tfms
• 126 localization files updated across 14 languages

Test Fixtures

• FallbackProjectParserTests.cs: SDK markers 13.0.0 → 13.1.0
• NewUpAndBuildSupportProjectTemplatesTests.cs: AspireVersionNext constant updated

Preserved by Design

• Template source directories remain ./13.0 (no 13.1 directories created)
• 9.5 version choice retained in templates
• MajorVersion unchanged (13)
• All package dependencies unchanged
• Historical 13.0 references in comments preserved

Build and template validation passing.

This pull request was created as a result of the following prompt from Copilot chat.

Update .NET Aspire repository branding from version 13.0 to 13.1, focusing ONLY on branding/version string updates (no structural template directory churn, dependency version changes, or additional template logic changes). Scope:

Repository: dotnet/aspire
Base branch: main

Goals:

1. Versioning Properties:
   • In eng/Versions.props: Update MinorVersion (currently 0) to 1 so that generated package versions become 13.1.0-* instead of 13.0.0-*.
   • Do NOT change MajorVersion (remains 13). Do NOT modify any other package/dependency versions or OpenTelemetry versions.
2. Milestone Automation:
   • In .github/policies/milestoneAssignment.prClosed.yml update milestone for merges to main from 13.0 to 13.1.
   • Leave release/* branch milestone rules untouched.
3. Template Branding (minimal):
   • For each template under src/Aspire.ProjectTemplates/templates/** that currently lists AspireVersion choices (e.g. 13.0 and 9.5): Replace occurrences of the current branding value 13.0 with 13.1 in:
     a. symbols.AspireVersion choices ("choice", "displayName", "description").
     b. Default value if set to 13.0 -> 13.1.
     c. Generated symbol logic (AspireVersionActual cases where falling back to 13.0 -> 13.1).
   • Do NOT add or remove previous version entries (keep 9.5 as previous even though this is logically outdated). This is intentional for this PR per user request to focus on branding only.
   • Do NOT copy or create new content directories (keep existing 13.0 directory; do NOT create a 13.1 directory; do NOT delete 9.5). Only update JSON/config string literals referencing 13.0.
4. Localization Files:
   • Update all localized template string files (.json in .template.config/localize/) that reference Aspire 13.0 (e.g. "Aspire 13.0" or choices/13.0) to 13.1 consistently.
5. Tests & Source Mentions:
   • Update any test data or code comments that explicitly reference 13.0 branding (e.g. single-file app host markers #:sdk Aspire.AppHost.Sdk@13.0.*) ONLY where the intent is to assert current branding. Avoid changing historical/version-specific test scenarios meant to check older versions; if uncertain, leave unchanged.
6. Docs (minimal):
   • Search for top-level documentation pages referencing "Aspire 13.0" as current version branding (if any new docs were introduced for 13.0). Update those literal phrases to "Aspire 13.1" ONLY if they are generic branding statements—not release notes tied specifically to 13.0 changes. Skip release notes or historical upgrade guides.
7. Do NOT modify dependency versions, placeholders, or add new features. Keep all non-branding strings intact.
8. Ensure build succeeds after changes.

Validation Steps (coding agent):

• Grep repository for "13.0" after changes; ensure only historical contexts (release notes, older upgrade guides) retain 13.0.
• Run dotnet build / pack for templates to verify no JSON schema errors.
• Run template-related tests if fast; otherwise note follow-up.

Deliverables:

• New branch off main with changes.
• Pull request titled "Update Aspire branding references from 13.0 to 13.1 (branding only)".
• PR description summarizing scope, explicit note that template directory lifecycle step (creating 13.1 content folder and locking 13.0) intentionally deferred.
• Checklist in PR body for reviewers confirming limited scope.

Non-Goals:

• No directory restructuring or lifecycle step execution.
• No dependency upgrades.
• No milestone changes beyond main branch rule.

Please implement changes carefully to avoid partial replacements inside version ranges or historical release notes.

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