POST /-/subscriptions creates OAuth token w/ expiration

What does this MR do and why?

• When a POST request is made to /-subscriptions we call GitlabSubscriptions::CreateService , which then creates an OAuth Access Token
• Access token created here: https://gitlab.com/gitlab-org/gitlab/-/blob/c4105d26999da65f070bce3b7a329f5ee3e79acc/ee/app/controllers/subscriptions_controller.rb#L104-109
• In theory, all OAuth tokens created in GitLab should have an expiration so this logic was unexpected.
• Removed support for OAuth tokens without expiration here: #340848 (closed)
• We will backfill expires_in for all of these tokens but first we need set a default expires_in
• It is possible that these OAuth tokens don't even work. So perhaps this OAuth token flow will be removed in the future via customers-gitlab-com#4682
• I've asked #g_fulfillment_platform about this and they said 2 hours is a safe default: https://gitlab.slack.com/archives/C037WQ2BUGK/p1669935926384809

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