Update link colors to use design token

What does this MR do and why?

Use design token gl-text-link instead of color assignment gl-text-blue-500 / gl-text-blue-600

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
Job log
Pipeline graph
Pipeline overview
Environments
MR sticky header
MR squash widget
Stat anchors
Help page
Vulnerability Report dropdown

How to set up and validate locally

1. Check GlLink and regular a links

Some test links:

2. Regular links can be seen e.g. for legacy timeline entries (like the system note for adding a commit message in an MR): http://gdk.test:3000/flightjs/Flight/-/merge_requests/4
3. The replies toggle can also be found in issuables/MRs for threads
4. Job details sidebar, e.g. http://gdk.test:3000/flightjs/Flight/-/jobs/563
5. Pipeline graph (linked pipeline, url): http://gdk.test:3000/flightjs/Flight/-/pipelines/9
6. Environment: http://gdk.test:3000/flightjs/Flight/-/environments/
7. Merge request sticky header: http://gdk.test:3000/flightjs/Flight/-/merge_requests/4
8. Design management:
   • View an issuable with a design attached as work item by replacing issues in the url with work_items
   • Click on the design, add a comment with a reply
   • Collapse replies
9. Anchor stats (Project overview): http://gdk.test:3000/flightjs/Flight

Related to #469616 (closed), #474921 (closed)