docs: Update contact.md with new Slack join URL

[metahertz]

Context

Update the community slack link to the new slack invitation url https://goto.prowler.com/slack

Does not fix a raised issue, just housekeeping.

Description

Single Docs URL change for the contact.md page.

Checklist

• Are there new checks included in this PR? No
• [N/A] Review if the code is being covered by tests.
• [N/A] Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• [N/A] Review if backport is needed.

License

No licence changes.
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jfagoagas]

Can you also update the README? Thanks!

[MrCloudSec]

Can you also update the README? Thanks!

and in the developer-guide/introduction.md, please!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.92%. Comparing base (bf4db86) to head (ca88c51).
Report is 27 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5671      +/-   ##
==========================================
+ Coverage   89.87%   89.92%   +0.04%     
==========================================
  Files        1119     1124       +5     
  Lines       34724    34815      +91     
==========================================
+ Hits        31209    31306      +97     
+ Misses       3515     3509       -6     

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[metahertz]

Squashed commits to one change, all references replaced. Confirmed with recursive grep for join.slack.

[jfagoagas]

Thank you @metahertz !!