Add deprecation entry for changing shared to specific runners

data/deprecations/14-5-deprecate-convert-instance-runner-to-project.yml

+- name: "Converting an instance (shared) runner to a project (specific) runner is deprecated"
+  announcement_milestone: "14.5" # The milestone when this feature was first announced as deprecated.
+  announcement_date: "2021-11-22"
+  removal_milestone: "15.0" # the milestone when this feature is planned to be removed
+  body: | # Do not modify this line, instead modify the lines below.
+    In GitLab 15.0, we will remove the feature that enables you to convert an instance (shared) runner to a project (specific) runner. Users who need to add a runner to only a particular project can register a runner to the project directly.
+
+  stage: Verify
+  tiers: [Core, Premium, Ultimate]
+  issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/345347
+

doc/update/deprecations.md

@@ -58,6 +58,12 @@ dramatically slow down GitLab instances. For this reason, they are being removed
 
 Announced: 2021-09-22
 
+### Converting an instance (shared) runner to a project (specific) runner is deprecated
+
+In GitLab 15.0, we will remove the feature that enables you to convert an instance (shared) runner to a project (specific) runner. Users who need to add a runner to only a particular project can register a runner to the project directly.
+
+Announced: 2021-11-22
+
 ### Deprecate `Versions` on base `PackageType`
 
 As part of the work to create a [Package Registry GraphQL API](https://gitlab.com/groups/gitlab-org/-/epics/6318), the Package group deprecated the `Version` type for the basic `PackageType` type and moved it to [`PackageDetailsType`](https://docs.gitlab.com/ee/api/graphql/reference/index.html#packagedetailstype).