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 📊

✅ 96014 passed | ⏭️ 6397 skipped | Total: 102411 | Pass Rate: 93.75% | Execution Time: 340m 47s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -2903
Passed Tests	📉 -2544
Failed Tests	—
Skipped Tests	📉 -359

All tests are passing successfully.

✅ Patch coverage is 94.87%. Project has 2486 uncovered lines.
❌ Project coverage is 89.72%. 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    89.83%    89.72%    -0.11%
==========================================
  Files          193       192        -1
  Lines        24588     24190      -398
  Branches      8744      8600      -144
==========================================
+ Hits         22089     21704      -385
- Misses        2499      2486       -13
- Partials      1400      1395        -5

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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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