Changed README to reflect new community forum

[dwmcallister]

Since we have discontinued the Slack NGINX community, I have updated the README to point to the NGINX Community Forum and specifically to the "Agent" tag section

Proposed changes

Updated to remove Slack reference and replace with Forum

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [ x] I have read the CONTRIBUTING document
• [ x] I have run make install-tools and have attached any dependency changes to this pull request
• [ x] If applicable, I have added tests that prove my fix is effective or that my feature works
• [ x] If applicable, I have checked that any relevant tests pass after adding my changes
• [ x] If applicable, I have updated any relevant documentation (README.md)
• [ x] If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[dwmcallister]

I have hereby read the F5 CLA and agree to its terms

[oCHRISo]

recheck