ci(benchmarks): add tenacity to flask benchmarks requirements file [backport 1.15] (#6078)

Backport 0fcd82e from #6071 to 1.15.

#5998 dropped tenacity as a package dependency, but our flask benchmark
utility methods use tenacity, making our flask benchmark jobs fail after
that PR was merged. We didn't catch this at the time of merge as that
was a forked PR meaning that our benchmark jobs did not run.

This PR adds tenacity back as a requirement for the flask scenarios
specifically.

Note: the flask-sqi and flask-simple benchmark PR jobs will fail here as
the baseline (`1.x`) branch will still have the same problem until after
this PR is merged.

## 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/)).

## 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.

Co-authored-by: Yun Kim <[email protected]>

Author: github-actions[bot]

benchmarks/flask_simple/requirements_scenario.txt

@@ -1,3 +1,4 @@
 flask==2.3.2
 gunicorn==20.1.0
 requests==2.31.0
+tenacity==8.0.0

benchmarks/flask_sqli/requirements_scenario.txt

@@ -1,3 +1,4 @@
 flask==2.3.2
 gunicorn==20.1.0
 requests==2.31.0
+tenacity==8.0.0