.github: update python version in render worflow

The script has an explicit dependency on ghapi package that implicitly
depends on fastcore and fetches the latest. Any python version older
than 3.12 leads to a failure while importing the package in the script
with the below error:

```
Traceback (most recent call last):
  File "/home/runner/work/redpanda/redpanda/./rpchangelog/rpchangelog.py", line 10, in <module>
    from ghapi.all import GhApi
  File "/opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/site-packages/ghapi/all.py", line 1, in <module>
    from .core import *
  File "/opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/site-packages/ghapi/core.py", line 9, in <module>
    from fastcore.all import *
  File "/opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/site-packages/fastcore/all.py", line 9, in <module>
    from .script import *
  File "/opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/site-packages/fastcore/script.py", line 15, in <module>
    from .docments import docments
  File "/opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/site-packages/fastcore/docments.py", line 359
    sig_str = f"def {get_name(self.obj)}(\n    {'\n    '.join(lines)}\n{self._ret_str}"
```

Author: gousteris

.github/workflows/render-pr-body-release-notes.yml

@@ -24,7 +24,7 @@ jobs:
           chmod +x rpchangelog/rpchangelog.py
       - uses: actions/setup-python@v5
         with:
-          python-version: '3.10'
+          python-version: '3.12'
           cache: 'pip'
       - run: pip3 install -r ./rpchangelog/requirements.txt
       - name: Render PR body release notes to job summary