Update CLA workflow file

[akshay-ap]

Context / issue

CLA workflow file is outdated and no in-line with other Safe repos.

Proposed solution

Update CLA workflow as per here: https://github.com/5afe/safe-core-protocol-specs/blob/main/.github/workflows/cla.yml

[mmv08]

What is exactly not in line? they seem almost identical to me

[mmv08]

Ahh, it's not using a remote repository to store the agreements.