Remove support for python 3.8 (#17908)

### Pull Request Checklist

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before submitting your pull request -->

* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
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The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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* [X] [Code
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correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

.github/workflows/tests.yml

@@ -397,7 +397,7 @@ jobs:
     needs:
       - linting-done
       - changes
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
 
@@ -409,12 +409,12 @@ jobs:
       # their build dependencies
       - run: |
           sudo apt-get -qq update
-          sudo apt-get -qq install build-essential libffi-dev python-dev \
+          sudo apt-get -qq install build-essential libffi-dev python3-dev \
             libxml2-dev libxslt-dev xmlsec1 zlib1g-dev libjpeg-dev libwebp-dev
 
       - uses: actions/setup-python@v5
         with:
-          python-version: '3.8'
+          python-version: '3.9'
 
       - name: Prepare old deps
         if: steps.cache-poetry-old-deps.outputs.cache-hit != 'true'
@@ -458,7 +458,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["pypy-3.8"]
+        python-version: ["pypy-3.9"]
         extras: ["all"]
 
     steps:
@@ -580,7 +580,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - python-version: "3.8"
+          - python-version: "3.9"
             postgres-version: "11"
 
           - python-version: "3.11"