fix(readme): replace broken Discord badge and update invite link

[Olexandr88]

Summary by CodeRabbit

• Documentation
  • Updated the Discord badge in the README to use a shields.io badge and link to a new invite URL. The previous badge image and invite destination have been replaced.

[github-actions]

This PR targeted main, which is only the default/static branch.

I retargeted it to v2-dev, the active development branch. Pushes to v2-dev automatically fast-forward main.

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