ref: Remove contextvars compatibility code in new major

[sentrivana]

Description

Drop the contextvars compatibility layer that supported Python 3.6 (aiocontextvars), old gevent (<20.9.0), and old greenlet (<0.5).

Warn if using an older version of either during setup.

This allows us to remove the whole machinery around checking whether contextvars are safe to use and replacing them if not.

Note: 3.6 tests are expected to fail on this. They'll be dropped in #6904

Issues

Closes #5037

Reminders

• Please add tests to validate your changes, and lint your code using uv run ruff.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[github-actions]

Codecov Results 📊

✅ 89524 passed | ❌ 1 failed | ⏭️ 5441 skipped | Total: 94966 | Pass Rate: 94.27% | Execution Time: 331m 25s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -6836
Passed Tests	📉 -6039
Failed Tests	📈 +1
Skipped Tests	📉 -798

➕ New Tests (1)

View new tests

• test_continuous_profiler_auto_start_and_stop_sampled_span_streaming[experiment-thread]
  • File: tests.profiler.test_continuous_profiler
  • Status: ❌ Failing

❌ Failed Tests

test_continuous_profiler_auto_start_and_stop_sampled_span_streaming[experiment-thread]

File: tests.profiler.test_continuous_profiler
Suite: py3.14t-common
Error: AssertionError: profiler should not be running assert '94fcf5220e9f41f5aca1a84cea6f200d' is None + where '94fcf5220e9f41f5aca1a84cea6f200d' = get_profiler_id()

Stack Trace

tests/profiler/test_continuous_profiler.py:896: in test_continuous_profiler_auto_start_and_stop_sampled_span_streaming
    assert get_profiler_id() is None, "profiler should not be running"
E   AssertionError: profiler should not be running
E   assert '94fcf5220e9f41f5aca1a84cea6f200d' is None
E    +  where '94fcf5220e9f41f5aca1a84cea6f200d' = get_profiler_id()

✅ Patch coverage is 94.87%. Project has 2343 uncovered lines.
✅ Project coverage is 90.09%. Comparing base (base) to head (head).

Files with missing lines (1)

File	Patch %	Lines
sentry_sdk/_init_implementation.py	88.89%	⚠️ 2 Missing and 2 partials

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.91%    90.09%    +0.18%
==========================================
  Files          193       186        -7
  Lines        24825     23654     -1171
  Branches      8912      8538      -374
==========================================
+ Hits         22321     21311     -1010
- Misses        2504      2343      -161
- Partials      1414      1368       -46

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

There are 2 total unresolved issues (including 1 from previous review).

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