make color of update-heading in primary-text-color

[basnijholt]

This fixes basnijholt/lovelace-ios-dark-mode-theme#47.

Breaking change

Color of the update-heading div will be editteable.

Proposed change

Make it primary-text-color.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Will fix the Update available: "Home Assistant Core 0.109.3" text color.

Additional information

• This PR fixes or closes issue: fixes Issue with "Update Available" with HassIO >0.109  basnijholt/lovelace-ios-dark-mode-theme#47
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

[bramkragten]

Thanks!