Astro 5.0 breaking changes tracker

[sarah11918]

📋 Explain your issue

This issue is to track 5.0 breaking changes to be part of the major version upgrade guide.

REMINDER: Only breaking changes need to be reported here. New features that do not break existing projects are cool! But they are not breaking changes and are not a part of the upgrade guide.

(See #5390 for v4.0 breaking changes tracker for past examples to follow)

You know the drill, use the following template:

### [changed/removed/added/deprecated] feature/name/title/description
In Astro v4.x, [ a statement in the past tense about what Astro did/used to do. 
e.g.  bundled all assets into several different folders....]

Astro v5.0 [ 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..

PR: #9999 (implementation)
PR: #5555 (docs, if appropriate)

Example upgrade guide: https://docs.astro.build/en/guides/upgrade-to/v5/

EMOJI LEGEND 👍 - this has been added to our Astro 5.0 Upgrade Guide #5481 😄 - NO CORRESPONDING DOCS CHANGES NEEDED (not a user-facing change) 🚀 - Docs need updating because of this change 🎉 - Docs PR for needed changes exists :-1: - Not actually a BREAKING change! Doesn't need to be listed in the guide!