refactor: fix @readonly property errors

[paulbalandan]

Description
Removes the @immutable class annotation from some config classes. The fact that their usage suggests they are not really immutable (and thus their properties not readonly) means the placement of the PHPDoc is wrong.

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