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

[bshaffer]

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)

[mkruskal-google]

Looks like you have some test failures on 32-bit and aarch64 :(

[bshaffer]

@mkruskal-google all passing now!

[anandolee]

Looks like still failing?

[bshaffer]

@anandolee no, they're passing.. looks like the job was stuck

[bshaffer]

@mkruskal-google one thing worth consideration - the calls to GPBUtil::check... have been removed for bool, float, and double. That means the following methods are no longer being called:

    public static function checkFloat(&$var)
    {
        if (is_float($var) || is_numeric($var)) {
            $var = unpack("f", pack("f", $var))[1];
        } else {
            throw new \Exception("Expect float.");
        }
    }

    public static function checkDouble(&$var)
    {
        if (is_float($var) || is_numeric($var)) {
            $var = floatval($var);
        } else {
            throw new \Exception("Expect float.");
        }
    }

    public static function checkBool(&$var)
    {
        if (is_array($var) || is_object($var)) {
            throw new \Exception("Expect boolean.");
        }
        $var = boolval($var);
    }

• in checkBool, the bool typehint should exhibit the same behavior thanks to the is_array($var) || is_object($var) check
• in checkFloat, the is_float and is_numeric will perform the same as the typecheck, but the unpack("f", pack("f", $var))[1]; is converting the 64-bit PHP float to a 32-bit float. That needs to be preserved.
• in checkDouble, the floatval result should exhibit the same behavior as the typehint

[bshaffer]

I have added back checkFloat, as the casting to a 32-bit float is critical. The other two (checkBool and checkFloat) aren't doing anything important

breaking change implication

Anyone who is running gencode in strict mode (e.g. declare(strict_types=1);) will experience a breaking change, as the float and bool type hints will reject everything outside those types (such as numeric and boolean strings). However, I believe this is WAI