Fix master CI after merging 6.x

[danog]

Fixes the CI failures on master after merging 6.x (895a33d):

• Unit tests (PropertyTypeTest): the merge kept master's `$assign_var->class instanceof Name` restriction in `AssignmentAnalyzer`, dropping that part of Check $class::$prop reads and assignments when the class is a variable #12040, so `$class::$prop = ...` was again not analyzed. The restriction is removed.
• Unit tests (AsymmetricVisibilityTest) / Psalm self-analysis: the new visibility check called `Properties::propertyExists()` with the 6.x signature. It now goes through `Codebase::propertyExists()`.
• Self-analysis: `StaticPropertyFetchAnalyzer::getClassNamesFromClassStringType()` is marked `@psalm-capabilities read-props`.
• PHPCS / stubs composer build: removed unused `md5` imports and sorted the use statements.
• Windows unit tests (failing since Print the purity of closures and callables as Closure[pure] and callable[impure] #12042): two `PurityTemplateTest` expected messages hardcoded `src/somefile.php`; they now use `DIRECTORY_SEPARATOR`.

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