Support for PHP 8.4

[erikfrerejean]

Description

Running Silverstripe 5.3 on a PHP 8.4 instance throws various deprecation errors, the issues identified so far:

• https://wiki.php.net/rfc/deprecations_php_8_4#remove_e_strict_error_level_and_deprecate_e_strict_constant, E_STRICT is a legacy error message constant which is now deprecated. This flag hasn't had any real use since PHP7 so we can remove the flag.
• https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated, implicit nullable parameters are deprecated. This can easily be resolved by adding a correct typehint on all properties.

Additional context or points of discussion

At the moment the CI doesn't test on PHP 8.4 yet

Related MRs

• WIP: Add PHP 8.4 support #11485

Validations

• You intend to implement the feature yourself
• You have read the contributing guide
• You strongly believe this feature should be in core, rather than being its own community module
• You have checked for existing issues or pull requests related to this feature (and didn't find any)

[GuySartorelli]

Closed in favour of silverstripe/.github#331

PHP 8.4 support will not be provided for CMS 5 as per the major release policy.