Fixed #1067 Community channel update to slack

[AmitGiriDev]

Description

Community channel updated from gitter to Slack

Related Issue

closes #1067

Motivation and Context

This change is needed as we changed community platform from gitter to Slack for communication.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.