refactor: Prevent illegal updates with KnoraUserService

[seakayone]

• Make UserChangeRequest private
• Expose use cases like updating SystemAdmin and UserStatus with dedicated methods

Pull Request Checklist

Task Description/Number

Issue Number: DEV-

PR Type

• build/chore: maintenance tasks (no production code change)
• docs: documentation changes (no production code change)
• feat: represents new features
• fix: represents bug fixes
• perf: performance improvements
• refactor: represents production code refactoring
• test: adding or refactoring tests (no production code change)

Basic requirements for bug fixes and features

• Tests for the changes have been added
• Docs have been added / updated

Does this PR introduce a breaking change?

• Yes

Does this PR change client-test-data?

• Yes

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 12.92%. Comparing base (eac5751) to head (04f6518).
Report is 113 commits behind head on main.

Files	Patch %	Lines
...api/slice/admin/api/service/UsersRestService.scala	0.00%	8 Missing ⚠️
.../slice/admin/domain/service/KnoraUserService.scala	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3098      +/-   ##
==========================================
+ Coverage   11.74%   12.92%   +1.18%     
==========================================
  Files         246      260      +14     
  Lines       22907    22339     -568     
==========================================
+ Hits         2690     2887     +197     
+ Misses      20217    19452     -765     

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[mpro7]

Just one minor suggestion:

[mpro7]

Suggested change

	UserChangeRequest(username = username, email = email, givenName = givenName, familyName = familyName, lang = lang)
	UserChangeRequest(username, email, givenName, familyName, lang)