Astro 3.0 breaking changes tracker #3756
Description
📋 Explain your issue
This issue is to track 3.0 breaking changes to be part of the guide. Will add things to this through comments, using this template:
### [changed/removed/added/deprecated] feature/name/title/description
In Astro v2.x, [ a statement in the past tense about what Astro did/used to do. e.g. Astro bundled all assets into several different folders....]
Astro v3.0 [ a statement in the present tense about how Astro works now. e.g. now bundles all assets into one single folder located in... ]
#### What should I do?
[Active sentences in the imperative (command) tense, with code examples showing diffs as appropriate]
e.g. Update... / Remove... / Replace... / Create...Example upgrade guide: https://docs.astro.build/en/guides/upgrade-to/v2/