Fix PHP 7.4 compatibility: replace static return type in ScriptFileManager::buildDisabledJsScript()

Problem/Motivation

On PHP 7.4 + Drupal 9, the module fatals during install/config import with:
ParseError: syntax error, unexpected 'static' (T_STATIC) in
src/Service/ScriptFileManager.php. The return type : static requires PHP ≥ 8.0.
Many Drupal 9 sites still run on PHP 7.4; this is a regression for those environments.

Steps to reproduce

1. Environment example:
   • Drupal 9.x (e.g. 9.5.x)
   • PHP 7.4.x (CLI and FPM)
2. Install/Config import via Drush:
   

   drush si --existing-config -y

3. Observe fatal error:
   

   ParseError: syntax error, unexpected 'static' (T_STATIC) in .../src/Service/ScriptFileManager.php on line 77

Proposed resolution

Change the method return type from static to self and update the docblock. This restores PHP 7.4 compatibility while remaining valid on PHP 8+.

Remaining tasks

• (Optional) Add a simple test/build job on PHP 7.4 to prevent regressions for D9 users.

User interface changes

None.

API changes

None.

Data model changes

None.