Update contrib guide to swap slack for discord

[derrickmehaffy]

What does it do?

Changes the Slack to Discord in the contrib guide

Why is it needed?

Deprov of Slack

How to test it?

click the link

Related issue(s)/PR(s)

N/A

[codecov]

Codecov Report

Merging #10692 (80ebcca) into master (e63fb9d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10692   +/-   ##
=======================================
  Coverage   58.01%   58.01%           
=======================================
  Files         185      185           
  Lines        6431     6431           
  Branches     1398     1398           
=======================================
  Hits         3731     3731           
  Misses       2236     2236           
  Partials      464      464           

Flag	Coverage Δ
front	?
unit	58.01% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e63fb9d...80ebcca. Read the comment docs.

[Convly]

LGTM