Add downgrade compatibility CI

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

.github/workflows/Downgrade.yml

@@ -0,0 +1,19 @@
+name: Downgrade
+on:
+  pull_request:
+    branches:
+      - master
+    paths-ignore:
+      - 'docs/**'
+  push:
+    branches:
+      - master
+    paths-ignore:
+      - 'docs/**'
+jobs:
+  test:
+    name: Downgrade
+    uses: "SciML/.github/.github/workflows/downgrade.yml@v1"
+    with:
+      julia-version: "lts"
+    secrets: "inherit"