[upgrade guide] remove mentions of beta

[sarah11918]

Description (required)

This removes all mention of "beta 5" from the upgrade guide so that it conforms to a regular, major (stable) upgrade guide.

Also some tiny polishing.

TODO (either in this PR, or before the main 5.0.0 PR is merged)

• create the unmaintained snapshot of v4 docs
• remove the yelling instruction in the docs for Chris to create the unmaintained snapshot of the v4 docs

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en/guides/upgrade-to/v5.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

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🔄️	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]

LGTM!