squash! Add schema validation for the various files in .github

We can't pass allowUnusedKeywords: true to the Action, so we need to
just use the entire lax mode, alas.

If we don't, we get:

> Error: Keyword not supported: "x-intellij-enum-metadata" at https://json.schemastore.org/dependabot-2.0.json#

.github/workflows/github-schema-validation.yml

@@ -30,6 +30,7 @@ jobs:
         with:
           file: '.github/dependabot.yml'
           schema: 'https://json.schemastore.org/dependabot-2.0.json'
+          mode: 'lax'
 
       - name: '.github/actions'
         if: ${{ hashFiles('.github/actions/*/action.yml') != '' }}