docs: add about removal of deprecated Config items

[kenjis]

Description
Supersedes #7021
See #7000 (comment)

• update backward_compatibility_notes.rst

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide

[samsonasik]

Thank you @kenjis

[kenjis]

@lonnieezell @MGatner Are you okay with this?

[MGatner]

I will defer to @lonnieezell - I'm not sure why we would ever need to remove a deprecated Config item, other than the very rare collision of things.

[kenjis]

The reason is that there may be bugs (e.g., #4297 (comment)) that cannot be fixed, and removing them makes the code easier to read and maintain.

[lonnieezell]

Already been merged, but I don't have problems with this, I suppose.

[MGatner]

I don't think that "easier to read and maintain" is enough of a reason to remove a deprecation early. Definitely in the case of bugs - we have already discussed that allowance under our practice of "Semver".