Use gl-text-secondary instead.

feat(GlFormGroup): Refactor text-gl-muted

See merge request gitlab-org/gitlab-ui!4181



Merged-by: Lukas 'ai-pi' Eipert <leipert@gitlab.com>
Approved-by: Rahul Chanila <rchanila@gitlab.com>
Approved-by: Lukas 'ai-pi' Eipert <leipert@gitlab.com>
Reviewed-by: Lukas 'ai-pi' Eipert <leipert@gitlab.com>
Reviewed-by: Sascha Eggenberger <seggenberger@gitlab.com>
Reviewed-by: Rahul Chanila <rchanila@gitlab.com>
Co-authored-by: Sascha Eggenberger <seggenberger@gitlab.com>

# [80.7.0](gitlab-org/gitlab-ui@v80.6.1...v80.7.0) (2024-05-08)

### Features

* **GlFormGroup:** Refactor text-gl-muted ([5c569d9d](gitlab-org/gitlab-ui@5c569d9d))