Update documentation support links

[JackPGreen]

Update support channels in documentation:

• Google Group link removed

  • The Hazelcast Google User Group is read-only. Please post your questions on the Hazelcast Community Slack)

[codecov[bot]]

Codecov Report

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

Project coverage is 93.42%. Comparing base (a95844f) to head (5c9bc0a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1521 +/- ## ========================================== - Coverage 93.45% 93.42% -0.04% ========================================== Files 466 466 Lines 16628 16628 Branches 1350 1350 ========================================== - Hits 15539 15534 -5 - Misses 793 799 +6 + Partials 296 295 -1 ```

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