Docs: 4.2 OpenTelemetry output documents oauth2.* options that only shipped in 5.0

The 4.2 docs for the OpenTelemetry output plugin list the full `oauth2.*` option set, but those options don't exist in any 4.2 release. According to the release notes, they were added in 5.0.

- 4.2 docs: [Docs 4.2](https://docs.fluentbit.io/manual/4.2/data-pipeline/outputs/opentelemetry)
- 5.0 docs: [Docs 5.0](https://docs.fluentbit.io/manual/5.0/data-pipeline/outputs/opentelemetry)

I tried to enable the feature on v4.2.8 and got an `unknown configuration property 'oauth2.enable'`

So, I guess the docs need to be updated.

Thank you!

Compare the linked Docs 4.2 and Docs 5.0 OpenTelemetry output pages, then verify the 4.2 release notes referenced in the issue. Remove the oauth2.* options from the 4.2 documentation while preserving them in the 5.0 page, and confirm the 4.2 page no longer documents unsupported options.

Written by the indexing model from the issue text.