ci: update pyenv in testrunner [backport 1.15] (#6189)

Backport bc79c86 from #6186 to 1.15.

This should resolve the failing ci build of the testrunner image. We can
test this by running the docker build after the update.

## 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/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [X] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [X] Backport labels are set (if
[applicable](../docs/contributing.rst#release-branch-maintenance))

## 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 nxcessary.
- [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.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](../docs/contributing.rst#release-branch-maintenance)

Co-authored-by: Tahir H. Butt <[email protected]>

Author: github-actions[bot]

docker/Dockerfile

@@ -66,7 +66,7 @@ RUN curl https://sh.rustup.rs -sSf | \
     sh -s -- --default-toolchain stable -y
 
 # Install pyenv and necessary Python versions
-RUN git clone --depth 1 --branch v2.3.17 https://github.com/pyenv/pyenv "${PYENV_ROOT}" \
+RUN git clone --depth 1 --branch v2.3.21 https://github.com/pyenv/pyenv "${PYENV_ROOT}" \
   && cd /root \
   && pyenv local | xargs -L 1 pyenv install \
   && cd -