chore(*): remove manualLowercase & manualUppercase functions

The `manualLowercase` & `manualUppercase` functions were inspired by Google Caja
code which worked around Java issues where problems with `toLowerCase`
working differently in Turkish locale are well known[1]. In JavaScript
`String#toLowerCase` is defined in the ECMAScript spec and all implementations
are required to lowercase I in the same way, regardless of the current locale.
Differences may (and do) happen only in `String#toLocaleLowerCase`.

The mirroring of the Java workarounds in Caja was needed due to an old Rhino bug.
Rhino is a pre-Nashorn JavaScript interpreter written in Java and it used to
delegate `String.prototype.toLowerCase` to `java.lang.String.toLowerCase`. This
has since been long fixed.

Other libraries doing string normalization, like jQuery or DOMPurify don't
apply special lowercasing logic in a Turkish environment.

Therefore, the `manualLowercase` & `manualUppercase` logic is dead code in
AngularJS and can be removed.

Also, the `manualLowercase` & `manualUppercase` functions are incomplete; they
only lowercase ASCII characters which is different to native
`String#toLowerCase`. Since those functions are used in many places in the
library, they would break a lot of code. For example, the lowercase filter would
not lowercase Ω to ω but leave it as Ω.

[1] https://garygregory.wordpress.com/2015/11/03/java-lowercase-conversion-turkey/

Closes #15890
Ref #11387