Fix plurality and capitalization in the documentation

What does this MR do?

While investigating package-and-qa for !22387 (closed) I noticed a few pluralization and capitalization errors, and while verifying those, noticed a few more.

• https://docs.gitlab.com/ee/api/group_milestones#delete-group-milestone
• https://docs.gitlab.com/ee/api/milestones#delete-project-milestone
• https://docs.gitlab.com/ee/development/testing_guide/end_to_end/#testing-nightly-builds
• https://docs.gitlab.com/ee/development/testing_guide/end_to_end/#testing-staging
• https://docs.gitlab.com/ee/development/policies#scores-order-performance
• https://docs.gitlab.com/ee/development/documentation/workflow.html#how-to-update-the-docs
• https://docs.gitlab.com/ee/development/contributing/issue_workflow#issue-triaging
• https://docs.gitlab.com/ee/user/project/new_ci_build_permissions_model#types-of-users
• https://docs.gitlab.com/ee/user/project/new_ci_build_permissions_model#before-gitlab-812
• https://docs.gitlab.com/ee/user/project/repository/repository_mirroring#overview

Related issues

None

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [-] If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.