ci: test against officially supported terraform versions (#780)

Hashicorp supports the two latest minor releases of terraform:
https://support.hashicorp.com/hc/en-us/articles/360021185113

.github/workflows/test.yml

@@ -1,21 +1,26 @@
-name: test
+name: Test
 
 on:
   pull_request:
 
 jobs:
-  integration_tests:
+  integration:
+    name: Integration ${{ matrix.terraform }}
     permissions:
       id-token: write
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        terraform: [ v1.5.x, v1.6.x ]
+      fail-fast: false
     steps:
       - name: Checkout
         uses: actions/checkout@v4
       - name: Unshallow
         run: git fetch --prune --unshallow
       - uses: hashicorp/setup-terraform@v2
         with:
-          terraform_version: v1.5.x
+          terraform_version: ${{ matrix.terraform }}
           terraform_wrapper: false
       - name: Set up Go
         uses: actions/setup-go@v4