Move shared workflows into folder '.github/workflows/shared'

This helps to distinguish workflows shared with and called from other
repositories and workflows called in this repository.

Author: HannesWell

.github/workflows/callUpdateTarget.yml

@@ -18,7 +18,7 @@ on:
 
 jobs:
   update:
-    uses: eclipse-platform/eclipse.platform.releng.aggregator/.github/workflows/updateTarget.yml@master
+    uses: ./.github/workflows/shared/updateTarget.yml
     with:
       author: Eclipse Releng Bot <releng-bot@eclipse.org>
       path: 'eclipse.platform.releng.prereqs.sdk'

.github/workflows/pr-checks.yml

@@ -11,13 +11,13 @@ on:
 
 jobs:
   check-freeze-period:
-    uses: ./.github/workflows/verifyFreezePeriod.yml
+    uses: ./.github/workflows/shared/verifyFreezePeriod.yml
   check-merge-commits:
-    uses: ./.github/workflows/checkMergeCommits.yml
+    uses: ./.github/workflows/shared/checkMergeCommits.yml
   check-versions:
     # Run this check only for PRs from forks and not for changes from bots created on branches in this repository
     if: github.event.pull_request.head.repo.full_name != github.repository
-    uses: ./.github/workflows/checkVersions.yml
+    uses: ./.github/workflows/shared/checkVersions.yml
     with:
       botName: Eclipse Platform Bot
       botMail: platform-bot@eclipse.org