fix: Do not use Unicode functions on lower and upper functions because of type incompatibility with string

[inoorinoor]

Query like this select e from StringEntity e where lower(e.name) like lower(concat('%', :search, '%')) fails with type incompatibility with Utf8 and String
This happens because lower and upper function are implemented by Unicode functions.

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Query like this select e from StringEntity e where lower(e.name) like lower(concat('%', :search, '%')) fails with
120:2 - 120:2: Mismatch type argument #1, type diff: Utf8!=String (S_ERROR)] error because in lower and upper functions are used Unicode::ToLower and Unicode::ToUpper.

Issue Number: N/A

What is the new behavior?

• Implementation of lower is changed from Unicode::ToLower to String::AsciiToLower
• Implementation of lower is changed from Unicode::ToUpper to String::AsciiToUpper

Other information

Added test that produces bug