Maintenance and Bugfix release

[FEATURE] Added ModifyDownloadRegistrationCsvEvent PSR-14 event #1164
[BUGFIX] Set request in parseStringFluid() function #1159
[TASK] Check for existing event in checkConfirmRegistration() #1155
[TASK] Stabilize acceptance tests on GitHub actions
[TASK] Avoid usage of PHP list() function #1158
[TASK] Use ConfigurationManagerInterface instead of ConfigurationManager #1161
[TASK] Drop hint about own templates in notification module #1165
[TASK] Migrate phpunit config
[TASK] Fix warning in FluidStandaloneServiceTest
[DOCS] fix two broken links #1160 - thanks @sandrotanner
[DOCS] Added missing section about addons #1152