[LIMS-1980] Remove CAS-specific logic

[gfrn]

JIRA ticket: LIMS-1980

Summary:

Removes some CAS-specific logic from the authorisation/authentication logic

Changes:

• Remove CAS-specific auth/auth logic

To test:
This is deployed to https://ebic-scaup-test.diamond.ac.uk/, you can get a token by logging into https://ebic-scaup-test.diamond.ac.uk/, going to the storage/cookies tab in your browser's developer mode and copying it (in this case, it's the one named __Host-sh_auth)

• Send a request to any endpoint with a Keycloak token as your Authorization: Bearer token, check it goes through OK
• Repeat the request, but remove any letter from the token to make it invalid, check it fails
• If you have a JWT emitted by the SBLIMS team, check that it still works with SCAUP

[codecov]

Codecov Report

❌ Patch coverage is 25.92593% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.32%. Comparing base (7c8f16b) to head (4f7804d).

Files with missing lines	Patch %	Lines
src/scaup/auth/micro.py	13.04%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   94.59%   94.32%   -0.27%     
==========================================
  Files          42       42              
  Lines        1740     1746       +6     
==========================================
+ Hits         1646     1647       +1     
- Misses         94       99       +5     

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