Concourse: GitHub tokens review

[sj-williams]

Background

We have several unique github tokens (some PAT) that are in use by Concourse pipelines

cloud_platform_infrastructure_pr_git_access_token
github_token
how_out_of_date_are_we_github_token
authorized_keys_github_token
github_actions_secrets_token

We need to review all of these, ensure that they belong to / are created by the appropriate user. ie if they are concourse-bot GH user PATs, we should look at renaming the var to reflect this.

Alternatively, we could look at possibility of managing them all via GH Concourse App?

relates to #5748