Raise kotlin language version in plugin build.

[runningcode]

We only support AGP 7.4+ so we only need to support 1.8+ according to
this: https://kotlinlang.org/docs/gradle-configure-project.html#apply-the-plugin

📜 Description

💡 Motivation and Context

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[github-actions]

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## Unreleased

- Raise kotlin language version in plugin build ([#931](https://github.com/getsentry/sentry-android-gradle-plugin/pull/931))

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