Fix enabling shared runners on group

What does this MR do and why?

See https://www.loom.com/share/bd00b971405f4c3e881d5bba77c0abe8?sid=6b931414-4db4-4c7d-8995-52a02cb19773 (second half and error)

Fix enabling shared runners on group if override is allowed

In Re-enable shared runners on descendants (!124728 - merged) we started to update allow_descendants_override_disabled_shared_runners when enabling shared runners.

It turns out that it breaks the old validation, which was validating the fact of changing the field instead of the end state.

This MR fixes the validation and adds some tests.

A few notes

• I tried to make the change minimal to fix the urgent bug before we release 16.2
• I can not simply turn off the feature flag introduced in Re-enable shared runners on descendants (!124728 - merged) because the update method there isn't guarded by it.

Screenshots or screen recordings

Screen_Recording_2023-07-14_at_16.59.53

How to set up and validate locally

See the recording above.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.