chore: make mixed plugin imports warning instead error

Signed-off-by: Hellgren Heikki <heikki.hellgren@op.fi>

.changeset/mean-pillows-enter.md

@@ -10,3 +10,6 @@ the backstage architecture. This rule forces that:
 - No imports from frontend plugins to backend plugins or other frontend plugins.
 - No imports from backend plugins to frontend plugins or other backend plugins.
 - No imports from common plugins to frontend or backend plugins.
+
+The current recommended configuration is giving a warning for mixed imports. This is to be changed in
+the future to an error so please adjust your workspace accordingly.

packages/eslint-plugin/index.js

@@ -22,7 +22,7 @@ module.exports = {
         '@backstage/no-forbidden-package-imports': 'error',
         '@backstage/no-relative-monorepo-imports': 'error',
         '@backstage/no-undeclared-imports': 'error',
-        '@backstage/no-mixed-plugin-imports': 'error',
+        '@backstage/no-mixed-plugin-imports': 'warn',
       },
     },
   },