docs: replace gitter links with discord

[Selflocking]

resolved: https://github.com/casbin/casbin-website-v2/issues/179

[casbin-bot]

@nodece @Shivansh-yadav13 please review

[github-actions[bot]]

:tada: This PR is included in version 2.7.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: