Remove references to closed Discord channel

[sarthakaggarwal97]

Closes #3220

The Valkey Discord server has been closed. This changes the entry from the GitHub issue template and the contributing guide.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR removes Discord contact links from issue template configuration and contributor documentation, replacing the Discord reference in CONTRIBUTING.md with a Matrix community link.

Changes

Community Contact Link Updates

Layer / File(s)	Summary
Issue template contact links .github/ISSUE_TEMPLATE/config.yml	Discord contact link entry is removed from the contact_links list, leaving the remaining channels (Questions, Matrix, Slack, Documentation issue) intact.
Contributor documentation CONTRIBUTING.md	The "Get started" section updates the community contact link by replacing the Discord URL with a Matrix URL.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Remove references to closed Discord channel' clearly and specifically describes the main change: removing Discord references from configuration files due to the closed server.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The pull request description directly relates to the changeset, clearly explaining that the Discord server has been closed and describing the changes made to remove Discord references.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.70%. Comparing base (fdf13ca) to head (92d8149).
⚠️ Report is 1 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #3722      +/-   ##
============================================
+ Coverage     76.65%   76.70%   +0.05%     
============================================
  Files           162      162              
  Lines         80662    80662              
============================================
+ Hits          61830    61874      +44     
+ Misses        18832    18788      -44     

see 22 files with indirect coverage changes

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