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.

added documentation + 1 deleted label

marked the checklist item Apply the documentation label. as completed

	4 Warnings
	2fd2d2e5: This commit’s subject line is acceptable, but please try to reduce it to 50 characters.
	Please add a throughput label to this merge request.
	This merge request does not have any assignee yet. Setting an assignee clarifies who needs to take action on the merge request at any given time.
	This merge request does not refer to an existing milestone.

Generated by Danger

assigned to @eread

assigned to @ngaskill and unassigned @eread

@ngaskill I noticed a few minor plurality and capitalization changes. Will you review these?

assigned to @eread and unassigned @ngaskill

added typebug label

Setting devopsmonitor based on groupapm.

added devopsmonitor label

removed devopsmonitor label

Setting devopsmonitor based on groupapm.

added devopsmonitor label

changed milestone to %12.8

changed milestone to %12.7

added Technical Writing backstage [DEPRECATED] labels

resolved all threads

approved this merge request

merged

mentioned in commit 7a0b1cac

resolved all threads

added docsfix label

mentioned in commit 7816adee

added grouprespond label and removed 1 deleted label