Upgrade oauth2 gem to v 2.0.9

What does this MR do and why?

• I added a minimum version of 2.0.6 because the next move will be to un-vendor omniauth-google-oauth2 and use the latest version instead, which specifies 2.0.6 as the minimum version: https://github.com/zquestz/omniauth-google-oauth2/pull/429/files
• CHANGELOG shows mostly bug fixes and test coverage improvements between these versions: https://gitlab.com/oauth-xx/oauth2/-/blob/main/CHANGELOG.md
• Diff is noisy due to lots of docs changes: https://my.diffend.io/gems/oauth2/2.0.3/2.0.9

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