feat: drop support for opentofu v1.6 (#1030)

Removes support for opentofu v1.6.

See https://endoflife.date/opentofu

Author: jooola

.github/workflows/test.yml

@@ -47,8 +47,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - tool: opentofu
-            version: v1.6.x
           - tool: opentofu
             version: v1.7.x
           - tool: terraform