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

[consideRatio]

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.

      - 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