Investigate and set lower bounds of requirements for `dask-gateway` and `dask-gateway-server`

In https://github.com/dask/dask-gateway/pull/533/files#r852422223 @martindurant noted the value of lower bounds for the requirements to install `dask-gateway` and `dask-gateway-server`.

I think that is a good practice and sustainable to maintain if it is tested in the CI system. When lower bounds are established, one can test them in the automatically like this [in jupyterhub](https://github.com/jupyterhub/jupyterhub/blob/c07358a52653dd0f3bccface7a11c6d36696b90f/.github/workflows/test.yml#L132-L143).

```yaml
- name: Install Python dependencies
run: |
pip install --upgrade pip
pip install --upgrade . -r dev-requirements.txt

if [ "${{ matrix.oldest_dependencies }}" != "" ]; then
# take any dependencies in requirements.txt such as tornado>=5.0
# and transform them to tornado==5.0 so we can run tests with
# the earliest-supported versions
cat requirements.txt | grep '>=' | sed -e 's@>=@==@g' > oldest-requirements.txt
pip install -r oldest-requirements.txt
fi
```