[2.9.0 Release] [Doc] Update Dask-on-Ray dependencies for 2.9.0 and 2.8.0

[architkulkarni]

Why are these changes needed?

Updates the Dask-on-ray compatibility table based on the following instruction:

Get the latest compatible Dask version from here and create a PR to master to update the doc. The PR needs to be cherry picked to the release branch as well. The file is moved to doc/source/ray-more-libs/dask-on-ray.rst

The 2.8.0 update was missing; this PR includes it as well.

These Ray versions do not support Python 3.7, so we don't have an entry for Python 3.7 like we do for older Ray versions in the table.

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[architkulkarni]

Assigning @angelinalg as ray-docs codeowner.

[architkulkarni]

Looks right, merging