Use EXT: path for backend module CSS in TYPO3 14

[Copilot]

In TYPO3 14, rendering the eRecht24 backend module can fail with SystemResourceDefinitionNotFoundException because f:be.pageRenderer receives a CSS path generated via f:uri.resource() (including query params) that is rejected as a project path.
This PR switches the CSS reference to an extension resource path accepted by TYPO3’s system resource handling.

• Backend layout resource loading
  • Updated Resources/Private/Layouts/Backend.html to stop passing f:uri.resource() output into includeCssFiles.
  • Replaced it with a static EXT: resource reference.

<f:be.pageRenderer includeCssFiles="{0: 'EXT:er24_rechtstexte/Resources/Public/Css/BackendModule.css'}"
    includeJavaScriptModules="{0: '@erecht24/er24-rechtstexte/erecht24-module.js'}">
</f:be.pageRenderer>

• Behavioral impact
  • Keeps backend module asset inclusion intact while avoiding disallowed typo3conf/ext/...?... project-path resolution in TYPO3 14.

[LvckyAPI]

@copilot And is it still backward compatible up to TYPO3 v12?

Or up to which version?
Can you send me the link to the documentation for that change?