Clarify runner plan limits

What does this MR do and why?

This MR is a follow-up to !175505 (merged) to try and clarify how runner plan limits are enforced, due to some confusion witnessed during the https://gitlab.com/gitlab-com/gl-infra/production/-/issues/18994 incident.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• https://gitlab.com/gitlab-com/gl-infra/production/-/issues/18994+

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

Before	After

app/helpers/plan_limits_helper.rb

@@ -16,9 +16,9 @@ def plan_limit_setting_description(limit_name)
     when :ci_needs_size_limit
       s_('AdminSettings|Maximum number of needs dependencies that a job can have')
     when :ci_registered_group_runners
-      s_('AdminSettings|Maximum number of runners created or active per group in the trailing 7-day period')
+      s_('AdminSettings|Maximum number of runners created or active in a group during the past seven days')
     when :ci_registered_project_runners
-      s_('AdminSettings|Maximum number of runners created or active per project in the trailing 7-day period')
+      s_('AdminSettings|Maximum number of runners created or active in a project during the past seven days')
     when :dotenv_size
       s_('AdminSettings|Maximum size of a dotenv artifact in bytes')
     when :dotenv_variables