[1.x] fix(core): change caching logic for validation attributes

[DavideIadeluca]

Fixes #0000
The caching logic of the previous solution to make validation attributes translatable was flawed.

• getAttributeNames() always stored the first set of attribute names it built under the single key
  core.validation.extension_id_class_names
• Every later validator – regardless of its own rules, extension ID or the current locale – hit that same key and therefore received the wrong map of attribute names

Changes proposed in this pull request:

• change caching logic for validation attributes
• deprecate now unused const

A cache key might now look like this:

• cacheKey":"core.validation.attributes.de.Flarum\\Discussion\\DiscussionValidator
• core.validation.attributes.de.Flarum\\Post\\PostValidator

Reviewers should focus on:
I tried to get a test running which covers this but had crazy difficulties changing the forum lang in the first place

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)