refactor!: remove deprecated APIs

[vdusek]

Summary

Removes deprecated APIs as part of the v3 major release:

• DatasetClient.download_items() — deprecated alias of get_items_as_bytes().
• max_unprocessed_requests_retries and min_delay_between_unprocessed_requests_retries arguments of batch_add_requests (sync + async). The docstrings already said "Will be removed in next major release."
• exclusive_start_id argument of list_requests (sync + async) — replaced by cursor.

Also drops the now-unused import warnings from both resource client modules and the associated unit tests for the removed cursor/exclusive_start_id mutex check.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.36%. Comparing base (7a579bf) to head (b3901cd).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #799      +/-   ##
==========================================
+ Coverage   94.23%   94.36%   +0.13%     
==========================================
  Files          49       49              
  Lines        4941     4920      -21     
==========================================
- Hits         4656     4643      -13     
+ Misses        285      277       -8     

Flag	Coverage Δ
integration	90.38% <ø> (+0.04%)	⬆️
unit	82.96% <ø> (+0.08%)	⬆️

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