Rename spawn-based launchers

[awaelchli]

What does this PR do?

Follow up to #13405

After generalizing the _SpawnLauncher to incorporate more than just the "spawn" start method, we should now rename the classes. This can be done without backward compatibility because the APIs are not user facing.

This PR should be merged using the "Rebase & Merge" option in GitHub. There should be exactly two commits:

• Renaming the classes
• Renaming the files

Does your PR introduce any breaking changes? If yes, please list them.

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cc @Borda @justusschock @awaelchli @rohitgr7 @kaushikb11 @akihironitta

[codecov]

Codecov Report

Merging #13743 (1a09c44) into master (c3299d2) will decrease coverage by 10%.
The diff coverage is 90%.

❗ Current head 1a09c44 differs from pull request most recent head 54da436. Consider uploading reports for the commit 54da436 to get more accurate results

@@            Coverage Diff            @@
##           master   #13743     +/-   ##
=========================================
- Coverage      86%      76%    -10%     
=========================================
  Files         327      327             
  Lines       25560    25560             
=========================================
- Hits        21969    19521   -2448     
- Misses       3591     6039   +2448