Fix incorrect stable branch name for templates

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24962 the
stable branches were incorrectly changed to `stable-X-Y`. This commit
fixes this so we correctly use `X-Y-stable`.

.gitlab/issue_templates/Security developer workflow.md

@@ -20,7 +20,7 @@ Set the title to: `Description of the original issue`
 - [ ] Once the MR is ready to be merged, create MRs targetting the last 3 releases, plus the current RC if between the 7th and 22nd of the month.
     - [ ] At this point, it might be easy to squash the commits from the MR into one
     - You can use the script `bin/secpick` instead of the following steps, to help you cherry-picking. See the [secpick documentation]
-    - [ ] Create each MR targetting the stable branch `stable-X-Y`, using the "Security Release" merge request template.
+    - [ ] Create each MR targetting the stable branch `X-Y-stable`, using the "Security Release" merge request template.
     - Every merge request will have its own set of TODOs, so make sure to
       complete those.
 - [ ] Make sure all MRs have a link in the [links section](#links)

.gitlab/merge_request_templates/Security Release.md

@@ -15,7 +15,7 @@ targets master.
 ## Developer checklist
 
 - [ ] Link to the developer security workflow issue on `dev.gitlab.org`
-- [ ] MR targets `master`, or `stable-X-Y` for backports
+- [ ] MR targets `master`, or `X-Y-stable` for backports
 - [ ] Milestone is set for the version this MR applies to
 - [ ] Title of this MR is the same as for all backports
 - [ ] A [CHANGELOG entry](https://docs.gitlab.com/ee/development/changelog.html) is added without a `merge_request` value, with `type` set to `security`

bin/secpick

@@ -10,7 +10,7 @@ using Rainbow
 
 module Secpick
   BRANCH_PREFIX = 'security'.freeze
-  STABLE_PREFIX = 'stable'.freeze
+  STABLE_SUFFIX = 'stable'.freeze
   DEFAULT_REMOTE = 'dev'.freeze
   NEW_MR_URL = 'https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/new'.freeze
 
@@ -38,7 +38,7 @@ module Secpick
     end
 
     def stable_branch
-      "#{STABLE_PREFIX}-#{@options[:version]}".tap do |name|
+      "#{@options[:version]}-#{STABLE_SUFFIX}".tap do |name|
         name << "-ee" if ee?
       end.freeze
     end