ci(cla): store signatures on unprotected cla-signatures branch

GITHUB_TOKEN cannot push the signature ledger to protected main; move the
store to a dedicated branch so the CLA check can persist signatures.

Author: opensource-SantanderAI

.github/workflows/cla.yml

@@ -36,5 +36,5 @@ jobs:
         with:
           path-to-signatures: ".cla-signatures/v1/cla.json"
           path-to-document: "https://github.com/SantanderAI/cla/blob/main/CLA.md"
-          branch: main
+          branch: cla-signatures
           allowlist: dependabot[bot],opensource-SantanderAI