Add documentation about required Core changes when updating minimum WordPress version

[desrosj]

What?

When the minimum version of WordPress required for the plugin is raised, there is a change required in Core SVN to ensure the E2E test workflow does not break in any branch losing support with the version bump.

This adds a call out about this in the related section of the documentation.

Notes

This doesn't feel like the best place to add this, but wasn't sure about where to add a new page for "Raising minimum required version of WordPress". Most of this "Update the reference commit" section could be pulled out into that more general page, I think.

See Core Changeset 59221.

I've also gone and created Core-62488 to account for the bump in #67117.

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Co-authored-by: desrosj <desrosj@git.wordpress.org>
Co-authored-by: ndiego <ndiego@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[ndiego]

Looks good!

[Mamaduka]

Thanks, @desrosj!