ci: pin urllib3 to 1.26.15 for starlette to fix failing CI [backport #5766 to 1.12] (#5779)

Backport of #5766 to 1.12

With urllib3 2.0+ we're seeing IncompleteRead errors being thrown from
the Starlette tests. This is blocking all PRs on CI failures so
currently pinning.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines)
are followed.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] PR description includes explicit acknowledgement/acceptance of the
performance implications of this PR as reported in the benchmarks PR
comment.

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.

Author: ZStriker19

.github/workflows/test_frameworks.yml

@@ -316,6 +316,9 @@ jobs:
         run: pip install ../ddtrace
       - name: Install dependencies
         run: scripts/install
+      # with urllib3 2.0 we are seeing IncompleteRead errors, pinning while investigating root issue.
+      - name: Pin urllib3
+        run: pip install urllib3==1.26.15
       #Parameters for keyword expression skip 3 failing tests that are expected due to asserting on headers. The errors are because our context propagation headers are being added
       #test_staticfiles_with_invalid_dir_permissions_returns_401 fails with and without ddtrace enabled
       - name: Run tests