Add kotlinOptions jvmTarget to templates

[gmackall]

Adds jvmTarget to gradle templates.

Fixes #147185.

Interestingly, we already had this in many of our examples/tests, see search results here https://github.com/search?q=repo%3Aflutter%2Fflutter%20jvmTarget&type=code.

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
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[bartekpacia]

✅

[reidbaker]

Feel free to say this is out of scope for this pr but I think we should have a variable like we do for minimum api level that is referenced for java version support and use that variable for both the java and kotlin versions.

[bartekpacia]

This sounds like a good idea to me. Example:

android {
    namespace "com.example.example_huy"
    compileSdk flutter.compileSdkVersion
    ndkVersion flutter.ndkVersion

    compileOptions {
        sourceCompatibility flutter.jvmTargetVersion
        targetCompatibility flutter.jvmTargetVersion
    }

    kotlinOptions {
        jvmTarget = flutter.jvmTargetVersion
    }

    ...
}

but yeah, it can land in a different PR

[gmackall]

Feel free to say this is out of scope for this pr but I think we should have a variable like we do for minimum api level that is referenced for java version support and use that variable for both the java and kotlin versions.

I think this is a good idea. I filed #147932