feat(docgen): Automatically add deprecated messages to removed errors

[Princesseuh]

Description (required)

Astro developers keep not following my carefully crafted rules regarding removing errors. Which probably mean my rules are not so carefully crafted. This PR makes it so when an error is removed, a basic deprecated message is added automatically, so at least there's some indication that one could upgrade their Astro version to proceed further.

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Tracked Files

Locale	File	Note
en	reference/errors/env-invalid-variable.mdx	Source changed, localizations will be marked as outdated.
en	reference/errors/invalid-rewrite404.mdx	Source changed, localizations will be marked as outdated.
en	reference/errors/locals-not-serializable.mdx	Source changed, localizations will be marked as outdated.
en	reference/errors/markdown-content-schema-validation-error.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[delucis]

🫡 Thanks @Princesseuh!