minor #982 Improved bc check (acrobat)

This PR was merged into the 2.x branch.

Discussion
----------

Only run the BC check on PR's and use the PR base branch as target branch to check the BC breaks against, this won't make the BC check fail false-positive on 2.x pr's

Commits
-------

e182f74 Improved bc check

.github/workflows/backwards-compatibility.yml

@@ -0,0 +1,18 @@
+name: "BC check"
+
+on:
+    pull_request:
+
+jobs:
+    roave_bc_check:
+        name: "Roave BC check"
+        runs-on: ubuntu-latest
+        steps:
+            -   uses: actions/checkout@v2
+                with:
+                    fetch-depth: 0
+
+            -   name: Roave BC Check
+                uses: docker://nyholm/roave-bc-check-ga
+                with:
+                    args: --from=${{ github.event.pull_request.base.sha }}

.github/workflows/ci.yml

@@ -28,15 +28,6 @@ jobs:
             - name: Run phpunit
               run: vendor/bin/phpunit --verbose
 
-    roave_bc_check:
-        name: Roave BC Check
-        runs-on: ubuntu-latest
-        steps:
-            - uses: actions/checkout@v2
-
-            - name: Roave BC Check
-              uses: docker://nyholm/roave-bc-check-ga
-
     phpstan:
         name: PHPStan
         runs-on: ubuntu-latest