Mention Social Blade integration removal in 2021.4 release notes

[SeanPM5]

Proposed change

This integration was removed just 3 days ago and I think it was forgot to added to release notes.

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logo's and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase: Remove Social Blade integration (ADR-0004) core#48677
• Link to parent pull request in the Brands repository: Remove Social Blade integration (ADR-0004) brands#2383
• This PR fixes or closes issue:

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

[SeanPM5]

The Social Blade integration page is still live, so it seems like this is being removed from the 2021.5 release instead, my mistake.