Remove service account references for bot users

What does this MR do and why?

Current documentation says that bots are the same as service accounts and links to a doc on billable users. Based on the definition of services accounts, bots are explicitly not a service account. It seems like this documentation term existed before we actually released services accounts in 16.1.

Since the behavior will be different for bots, I have updated the documentation for project and group bots to state they are non-billable users rather than service accounts.

This came up while answer a customer's questions on access tokens. Since bots and service accounts are different, the behavior with access tokens will also be different. The current documentation does not make this clear.

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