add legacy.collectionsBackwardsCompat flag docs for v6

[matthewp]

Description (required)

Adds docs for the v6 collection legacy flag to:

• v6 upgrade guide
• Legacy flags page

For Astro version: 6.x.

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Tracked Files

File	Note
en/guides/upgrade-to/v6.mdx	Localization changed, will be marked as complete.
en/reference/legacy-flags.mdx	Source changed, localizations will be marked as outdated.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.

[sarah11918]

This looks great, @matthewp ! Just suggested some small edits to the wording, but this works very well!

[sarah11918]

(Noting that this feature was already released, so this PR is not merge-on-release. It can be merged into the v6 branch whenever!)

[sarah11918]

This looks fantastic @matthewp ! Go ahead and merge when you're happy!