Include branch name other than main in the update attribution PR title

[sp1999]

Description of changes: The most recent PR created by eks-distro-pr-bot to update ATTRIBUTION.txt files on release-0.20 branch did not include the branch name in the PR title. We follow the convention to prepend "[BRANCH_NAME] " to the PR title for most of the other automated PRs created by the bot when the PR is being created for any release branch in order to quickly identify the base branch. This PR updates the create_attribution function to follow this convention. The -r argument is removed for the pr_title local variable to allow updating it else it would be a read-only variable and throw an error.

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