[Meta] Plan for deprecating and eventually removing the super user access policy

Problem/Motivation

Now that the special powers tied to user 1 are neatly contained in an access policy, we can switch it off. This is what was made possible in #540008: Add a container parameter that can remove the special behavior of UID#1.

However, that issue only made sure that core tests run without the super user policy, proving that they have the correct permissions assigned in their setup. The updates to core tests are being tackled here: #3437620: [Meta] Fix all tests that rely on UID1's super user behavior

For contrib, we need to come up with a plan on how to trigger a deprecation warning when a tests runs without setting $usesSuperUserAccessPolicy to FALSE. That is being discussed here: #3437926: Decide on when/how we will run contrib/custom tests without the super user access policy

The end goal is to completely remove the super user access policy from core as it's a possible attack vector.

Finally, we need to devise a way to recover your website if you removed your own admin role by accident, because you'll no longer be able to rely on user 1 having permanent admin powers. This is being discussed here: #3438904: Introduce recovery tool or documentation for when someone is locked out of their own site

Steps to reproduce

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Proposed resolution

Resolve all linked issues in their issue, keep this issue as a parent for meta discussion.

Remaining tasks

Resolve all linked issues

User interface changes

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API changes

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Data model changes

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Release notes snippet

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