Update URLs to better ones

What does this MR do?

Fixed some URLs that were broken according to link checkers, usually because they were invalid or required redirects.

Related issues

Relates to: https://gitlab.com/gitlab-org/gitlab-ce/issues/51696

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards