build: Upgrade to Node 20

Add official Node 20 support by:

* Upgrading to Node 20 in `.nvmrc`
* Regenerating `package-lock.json` from scratch using it
* Blocking on Node 20 CI failures
* Updating lockfile check workflow to v3

.github/workflows/ci.yml

@@ -12,7 +12,6 @@ jobs:
     strategy:
       matrix:
         node: [18, 20]
-    continue-on-error: ${{ matrix.node == 20 }}
 
     steps:
     - name: Checkout

.github/workflows/lockfileversion-check.yml

@@ -10,4 +10,4 @@ on:
 
 jobs:
   version-check:
-    uses: openedx/.github/.github/workflows/lockfile-check.yml@master
+    uses: openedx/.github/.github/workflows/lockfile-check-v3.yml@master

.nvmrc

@@ -1 +1 @@
-18
+20