fix(Angular): remove `angular.lowercase` and `angular.uppercase`

Closes #15445

BREAKING CHANGE:

The helper functions `angular.lowercase` `and angular.uppercase` have
been removed.

These functions have been deprecated since 1.5.0. They are internally
used, but should not be exposed as they contain special locale handling 
(for Turkish) to maintain internal consistency regardless of user-set locale.

Developers should generally use the built-ins `toLowerCase` and `toUpperCase`
or `toLocaleLowerCase` and `toLocaleUpperCase` for special cases.

Further, we generally discourage using the angular.x helpers in application code.