fix: update Slack references to Discord

[anablaz]

Closes # https://github.com/accordproject/techdocs/issues/388

This repository was listed among the repos that need an update of Slack references to Discord.

Changes

• Update Slack references to Discord

Related Issues

• Issue # https://github.com/accordproject/techdocs/issues/388

Author Checklist

• [X] Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• [ ] Vital features and changes captured in unit and/or integration tests
• [X] Commits messages follow AP format
• [ ] Extend the documentation, if necessary
• [X] Merging to main from fork:branchname

[coveralls]

Pull Request Test Coverage Report for Build 8203728089

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 69.978%

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Totals
Change from base Build 6718925375:	0.0%
Covered Lines:	2785
Relevant Lines:	3811

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💛 - Coveralls