refactor: mark pkg/strings as deprecated in favor of stdlib

[isegura-eos-eng]

This attempts to solve Issue #679.

In summary, since this project enforces +go1.21 we can deprecate the pkg/strings package which only implements a similar function from stdlib: slices.Contains.

Definition of Ready

• [x ] I am happy with the code
• [ x] Short description of the feature/issue is added in the pr description
• [x ] PR is linked to the corresponding user story
• Acceptance criteria are met
• All open todos and follow ups are defined in a new ticket and justified
• Deviations from the acceptance criteria and design are agreed with the PO and documented.
• No debug or dead code
• My code has no repetitions
• Critical parts are tested automatically
• Where possible E2E tests are implemented
• Documentation/examples are up-to-date
• All non-functional requirements are met
• Functionality of the acceptance criteria is checked manually on the dev system.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 60.97%. Comparing base (0992c5f) to head (e308fa6).
Report is 107 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/op/auth_request.go	33.33%	2 Missing ⚠️
pkg/op/token_refresh.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #680      +/-   ##
==========================================
+ Coverage   60.06%   60.97%   +0.91%     
==========================================
  Files          80       81       +1     
  Lines        6998     7459     +461     
==========================================
+ Hits         4203     4548     +345     
- Misses       2498     2601     +103     
- Partials      297      310      +13     

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[muhlemmer]

Thanks for the housekeeping! Just one small comment.