chore(framework tests): pin django framework test ubuntu to 20.04 (#4636)

## Description
~Investigating the cause behind failing django framework tests. I can't reproduce the failures locally, so I'm trying on Github actions.~
The recent github actions upgrade to `ubuntu-latest` to use `ubuntu-22.04` instead of `ubuntu-20.04` is causing failures on the django framework tests. I've pinned the ubuntu version to `ubuntu-20.04` instead of `ubuntu-latest` to avoid this problem.

## Checklist
- [ ] Followed the [library release note guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines) when writing a release note.
- [ ] Add additional sections for `feat` and `fix` pull requests.
- [ ] [Library documentation](https://github.com/DataDog/dd-trace-py/tree/1.x/docs) and/or [Datadog's documentation site](https://github.com/DataDog/documentation/) is updated. Link to doc PR in description.

## Motivation

## Design

## Testing strategy

## Relevant issue(s)

## Testing strategy

## Reviewer Checklist
- [ ] Title is accurate.
- [ ] Description motivates each change.
- [ ] No unnecessary changes were introduced in this PR.
- [ ] Avoid breaking [API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces) changes unless absolutely necessary.
- [ ] Tests provided or description of manual testing performed is included in the code or PR.
- [ ] Release note has been added and follows the [library release note guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines), or else `changelog/no-changelog` label added.
- [ ] All relevant GitHub issues are correctly linked.
- [ ] Backports are identified and tagged with Mergifyio.

Author: Yun-Kim

.github/workflows/test_frameworks.yml

@@ -53,7 +53,7 @@ jobs:
         run: ddtrace-run pytest test --continue-on-collection-errors -v -k 'not TestServerAdapter_gunicorn'
 
   django-testsuite-3_1:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       DD_PROFILING_ENABLED: true
       DD_TESTING_RAISE: true