feat(php): Add PHP typehints for setters and remove redundant GPBUtil checks (#25296)

This PR modifies `php_generator.cc` to:
- Add PHP typehints to setter method signatures for all types (for 64-bit integers, the type hint is `int|string` to maintain compatibility with 32-bit systems)
- Remove redundant `GPBUtil::check...` for primitive types and messages (the PHP type hint provides sufficient validation)
- Retain `GPBUtil::check...` for 32 and 64-bit integers, strings (for UTF-8 validation), enums, maps, and repeated fields

Additionally makes the following changes:
- Updates descriptor protos with generator changes
- Suppresses float to int "loss of precision" warnings in tests which expect it
- Removes compatibility in tests with PHPUnit 6, which is no longer needed
- Proper casing in gencode for `true` and `false` (instead of `True` and `False`)

Closes #25296

PiperOrigin-RevId: 856990912

Author: bshaffer