Clean up app module build.gradle.kts

[maksym-moroz]

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• I have read how to contribute to this project
• I have signed the Contributor License Agreement

Contributor checklist

• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• Samsung Note 10+, Android 12

• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #1234 syntax

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Description

• Remove legacy compiler arg that no longer does anything https://github.com/Kotlin/KEEP/blob/master/proposals/stdlib/result.md#limitations-legacy
• Update deprecated setMinSdk()/setTargetSdk() to direct counterpart properties
• Remove variable that serves no purpose (optional)
• Refactor disabling variants to the modern beforeVariants {} lazy dsl
• Improve sourceSet srcDir setup to use lazy APIs

I made sure to test all the Gradle changes locally. All changes are split into separate commits and named accordingly.
Behaviour matches the previous version, checked by logs. Everything syncs, runs and builds as before (maybe better).

[greyson-signal]

Thank you! This will go out in our 6.46 release :)

[greyson-signal]

(FYI don't worry about the lint errors, already fixed them in our candidate branch)