[GOBBLIN-1761] Update Gobblin OSS Slack channel link to a never-expire link

[Will-Lo]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Gobblin JIRA issues and references them in the PR title. For example, "[GOBBLIN-XXX] My Gobblin PR"
  • https://issues.apache.org/jira/browse/GOBBLIN-XXX
    Updates the slack invite link

Description

• Here are some details about my PR, including screenshots (if applicable):
  Updates the slack link. It should never expire this time.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[codecov-commenter]

Codecov Report

Merging #3620 (1a73ff1) into master (5d1461f) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3620   +/-   ##
=========================================
  Coverage     46.54%   46.55%           
+ Complexity    10646    10644    -2     
=========================================
  Files          2129     2129           
  Lines         83291    83291           
  Branches       9279     9279           
=========================================
+ Hits          38768    38773    +5     
+ Misses        40960    40954    -6     
- Partials       3563     3564    +1     

Impacted Files	Coverage Δ
...lin/elasticsearch/writer/FutureCallbackHolder.java	61.42% <0.00%> (-1.43%)	⬇️
...a/management/copy/publisher/CopyDataPublisher.java	74.17% <0.00%> (-1.33%)	⬇️
.../org/apache/gobblin/cluster/GobblinTaskRunner.java	62.53% <0.00%> (+0.30%)	⬆️
.../apache/gobblin/runtime/api/JobExecutionState.java	80.37% <0.00%> (+0.93%)	⬆️
...a/org/apache/gobblin/cluster/GobblinHelixTask.java	66.66% <0.00%> (+2.15%)	⬆️
...lin/restli/throttling/ZookeeperLeaderElection.java	72.22% <0.00%> (+2.22%)	⬆️
.../gobblin/cluster/GobblinHelixTaskStateTracker.java	68.75% <0.00%> (+6.25%)	⬆️

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