build: fix docs build (#12881)

In PR #12634,
`.kokoro/docker/docs/Dockerfile` was updated to use `ubuntu:24.04`. The
previous version `ubuntu:22.04` had Python 3.10 installed by default. In
`ubuntu:24.04`, Python 3.12 is the default version. We need to restore
`Python 3.10` which is used in both the `docs` and `docfx` nox sessions.


https://github.com/googleapis/google-cloud-python/blob/81f6e2b4837e7174b2c7a8866ebba53eba25a3c7/packages/google-cloud-access-approval/noxfile.py#L36


https://github.com/googleapis/google-cloud-python/blob/81f6e2b4837e7174b2c7a8866ebba53eba25a3c7/packages/google-cloud-access-approval/noxfile.py#L290-L291


https://github.com/googleapis/google-cloud-python/blob/81f6e2b4837e7174b2c7a8866ebba53eba25a3c7/packages/google-cloud-access-approval/noxfile.py#L325-L326


I tested that this builds successfully by running `docker build -t
test_docs_mono_repo .` in the directory `kokoro/docker/docs`


I then ran the nox sessions `docs` and `docfx`.

Use this command to run the docker container and enter into bash

```
docker run --rm -it --entrypoint /bin/bash test_docs_mono_repo
```

Then in the docker container, run the following commands
```
git clone https://github.com/googleapis/google-cloud-python.git
cd google-cloud-python/packages/google-cloud-access-approval
nox -s docs
nox -s docfx
```

.kokoro/docker/docs/Dockerfile

@@ -40,7 +40,6 @@ RUN apt-get update \
     libssl-dev \
     libsqlite3-dev \
     portaudio19-dev \
-    python3-distutils \
     redis-server \
     software-properties-common \
     ssh \
@@ -60,24 +59,27 @@ RUN apt-get update \
   && rm -rf /var/lib/apt/lists/* \
   && rm -f /var/cache/apt/archives/*.deb
 
-###################### Install python 3.9.13
+###################### Install python 3.10.14 for docs/docfx session
 
-# Download python 3.9.13
-RUN wget https://www.python.org/ftp/python/3.9.13/Python-3.9.13.tgz
+# Download python 3.10.14
+RUN wget https://www.python.org/ftp/python/3.10.14/Python-3.10.14.tgz
 
 # Extract files
-RUN tar -xvf Python-3.9.13.tgz
+RUN tar -xvf Python-3.10.14.tgz
 
-# Install python 3.9.13
-RUN ./Python-3.9.13/configure --enable-optimizations
+# Install python 3.10.14
+RUN ./Python-3.10.14/configure --enable-optimizations
 RUN make altinstall
 
+RUN python3.10 -m venv /venv
+ENV PATH /venv/bin:$PATH
+
 ###################### Install pip
 RUN wget -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
   && python3 /tmp/get-pip.py \
   && rm /tmp/get-pip.py
 
 # Test pip
-RUN python3.9 -m pip
+RUN python3.10 -m pip
 
-CMD ["python3.9"]
+CMD ["python3.10"]