Fix slow SyntheticSource in Python3.11+, clean up Python load test version configs

[Abacn]

Fix #40509 ; fix #40510 ; fix #40511 ; fix #40513

• Avoid time.sleep(0) invocation in Synthetic Source that causing regression in Py311 (time.sleep(0) is slower on Python 3.11 than on Python 3.10 python/cpython#125997)

• Remove no-op "--experiments=use_runner_v2"

• Use snapshot SDK containers for load test, instead of default to beam-master. This makes regression in dependencies to be exposed earlier

[Abacn]

Testing:

https://github.com/apache/beam/actions/runs/38304879269 (Dataflow)

https://github.com/apache/beam/actions/runs/38304887892 (Flink)

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.72%. Comparing base (76f6e6d) to head (5d97388).
⚠️ Report is 139 commits behind head on master.

Files with missing lines	Patch %	Lines
...s/python/apache_beam/testing/synthetic_pipeline.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40540       +/-   ##
=============================================
+ Coverage     56.16%   79.72%   +23.55%     
=============================================
  Files          1124      487      -637     
  Lines        178336    82289    -96047     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    65601    -34569     
+ Misses        75645    16688    -58957     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	79.72% <50.00%> (+0.16%)	⬆️

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[github-actions]

Assigning reviewers:

R: @jrmccluskey for label python.

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