Remove Elemental MediaStore

[viren-nadkarni]

AWS retired Elemental MediaStore on 2025-11-13. It is no longer accessible on AWS console and the docs have also been deleted.

https://aws.amazon.com/blogs/media/support-for-aws-elemental-mediastore-ending-soon/

This PR removes 'mediastore' and 'mediastore-data' from Moto.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.04%. Comparing base (4c92564) to head (d4a0942).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9542      +/-   ##
==========================================
- Coverage   93.05%   93.04%   -0.02%     
==========================================
  Files        1301     1291      -10     
  Lines      117004   116764     -240     
==========================================
- Hits       108878   108640     -238     
+ Misses       8126     8124       -2     

Flag	Coverage Δ
servertests	28.82% <ø> (-0.04%)	⬇️
unittests	93.01% <ø> (-0.02%)	⬇️

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[bblommers]

Thanks @viren-nadkarni! Will merge it once botocore drops support for it 👍