ci: use 3.12.0 release in testrunner [backport 2.3] (#7859)

Backport 46dfab9 from #7780 to 2.3.

## 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](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## 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.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

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

Author: github-actions[bot]

docker/.python-version

@@ -3,4 +3,4 @@
 3.8.16
 3.9.16
 3.11.3
-3.12-dev
+3.12.0

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 v2.3.21 https://github.com/pyenv/pyenv "${PYENV_ROOT}" \
+RUN git clone --depth 1 --branch v2.3.33 https://github.com/pyenv/pyenv "${PYENV_ROOT}" \
   && cd /root \
   && pyenv local | xargs -L 1 pyenv install \
   && cd -