* Introduce new AuditEntryService
- Moved logic related to the IAuditEntryRepository from the AuditService to the new service
- Introduced new Async methods
- Using ids (for easier transition from the previous Write method)
- Using keys
- Moved and updated integration tests related to the audit entries to a new test class `AuditEntryServiceTests`
- Added unit tests class `AuditEntryServiceTests` and added a few unit tests
- Added migration to add columns for `performingUserKey` and `affectedUserKey` and convert existing user ids
- Adjusted usages of the old AuditService.Write method to use the new one (mostly notification handlers)
* Audit service rework
- Added new async and paged methods
- Marked (now) redundant methods as obsolete
- Updated all of the usages to use the non-obsolete methods
- Added unit tests class `AuditServiceTests` and some unit tests
- Updated existing integration test
* Apply suggestions from code review
* Small improvement
* Update src/Umbraco.Core/Services/AuditService.cs
* Some minor adjustments following the merge
* Delete unnecessary file
* Small cleanup on the tests
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