Add support for gevent - #6372

* Bump pip and setuptools and switch from setup.py to pip install

* Add gevent to requirements, bumps for compatibility and regenerate requirement files.

* Display which components fail unit tests.

* Add checks for eventlet code in monkey patch

- Took upstream's CI image bumps (py3.10, mongo 8.2, rabbitmq 4.2) in .circleci/config.yml
- Merged Makefile: kept setuptools_scm/python -m build (needed for gevent's setuptools pin), took upstream's $(PYBIN) consistency
- fixed-requirements.txt: kept gevent/eventlet/greenlet/gunicorn pins, dropped unused flex, picked up importlib_resources/importlib_metadata
- Regenerated all per-component requirements.txt via make requirements
- Regenerated lockfiles/st2.lock via pants generate-lockfiles --resolve=st2
- st2client httpclient.py: resolved shlex.quote alias naming
- test_service_setup_log_level_filtering.py: kept concurrency.sleep() abstraction, took upstream's SIGTERM/8s timeout flakiness fix

- Read concurrency_library from st2.conf [system] instead of an env var,
  defaulting to gevent.
- Pick the gunicorn worker class (-k eventlet/-k gevent) dynamically based
  on the configured concurrency library instead of hardcoding eventlet.
- Fix real gevent bugs: wrap_ssl() using the removed ssl.wrap_socket(),
  green_pool_wait_all() not actually blocking, wait() not returning the
  greenlet's result, and several call sites bypassing the concurrency
  wrapper with direct .wait()/.free() calls on greenlets/pools.
- Fix a gevent-only hang in the python runner: process.stdin was written
  to but never closed/flushed for large (>64KB) parameters, leaving the
  child process blocked forever waiting for input that never arrived.
- Remove remaining direct eventlet imports outside the concurrency
  compatibility layer (tools, examples, test mocks); keep back-compat
  aliases (EventletTestCase, setup_eventlet_profiler,
  blocking_eventlet_spawn) for external callers.