Remove legacy Google Groups mailing list from `README.rst`

[jakob-keller]

Description of Change

Remove legacy Google Groups mailing list from README.rst

Assumptions

Stakeholders know about and will use established channels, i.e. GitHub issues and discussions.

Checklist for All Submissions

• [ ] I have added change info to CHANGES.rst
• [ ] If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • [ ] Detailed description of issue
  • [ ] Alternative methods considered (if any)
  • [ ] How issue is being resolved
  • [ ] How issue can be reproduced
• [ ] If this is providing a new feature (can be provided via link to issue with these details):
  • [ ] Detailed description of new feature
  • [ ] Why needed
  • [ ] Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• [ ] I have read and followed CONTRIBUTING.rst
• [ ] I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• [ ] I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.03%. Comparing base (8d7c18f) to head (380eea1). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1159 +/- ## ======================================= Coverage 87.03% 87.03% ======================================= Files 64 64 Lines 5969 5969 ======================================= Hits 5195 5195 Misses 774 774 ``` | [Flag](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1159/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1159/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `87.03% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs#carryforward-flags-in-the-pull-request-comment) to find out more.

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