What does this PR change? What problem does it solve?

We already support forget --prune --no-lock --dry-run, but directly calling prune --no-lock --dry-run was not possible. Also allow this for the prune command.

Was the change previously discussed in an issue or on the forum?

No.

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.