Update XCMetrics plugin URL to new Backstage community plugins repository

[asoneji]

Description:

This PR updates the XCMetrics plugin URL from:
🔴 Old URL: https://github.com/backstage/backstage/tree/master/plugins/xcmetrics
🟢 New URL: https://github.com/backstage/community-plugins/tree/main/workspaces/xcmetrics/plugins/xcmetrics

The Backstage team has migrated community plugins to a new repository (community-plugins), and this change ensures the link points to the correct location.

Changes:

• Updated the XCMetrics plugin URL to the new community-plugins repository.

Why this change?

• The original link is outdated due to Backstage's repository restructuring.
• This ensures that contributors and users can find the latest version of the XCMetrics plugin in the correct location.

Testing & Validation:

• Verified that the new URL is accessible and correctly redirects to the XCMetrics plugin.

Additional Notes:

• No code functionality is affected by this change—only documentation and reference updates.