Handle missing GitHub resources

[czembower]

Description

Updates resource_github_auth_backend, resource_github_team, and resource_github_user to handle resources present in state but missing from Vault configuration.

For resource_github_auth_backend, authMountInfoGet was simply relocated to occur after checking for the existence of the mount. Comments inline for the other two changes.

Closes #2123

Output from acceptance testing:

=== RUN TestAccGithubTeam_basic
--- PASS: TestAccGithubTeam_basic (1.85s)
=== RUN TestAccGithubTeam_teamConfigError
--- PASS: TestAccGithubTeam_teamConfigError (0.12s)
=== RUN TestAccGithubTeam_importBasic
--- PASS: TestAccGithubTeam_importBasic (1.30s)
PASS

=== RUN TestAccGithubUser_basic
--- PASS: TestAccGithubUser_basic (1.70s)
=== RUN TestAccGithubUser_importBasic
--- PASS: TestAccGithubUser_importBasic (1.16s)
PASS

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[fairclothjm]

Thanks @czembower ! Sorry for the delay on this one! We have recently updated vault/resource_github_auth_backend.go which I think partially resolves your issue. Would you mind rebasing on main?