chore: upgraded Gradle to 8.7.1

[larssn]

Since Flutter now defaults to Gradle 8, I've taken the liberty of upgrading Gradle to 8.7.1.

Also ran through all the Gradle settings, and brought them up to date with the standard from a vanilla platform plugin.
Removed the Kotlin dependencies, as they are not needed.

Changed the build targets to Java 8, as minSdk 26 requires this:

compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
}

kotlinOptions {
    jvmTarget = "1.8"
}

This should result in "correct" bytecode, which is also why Flutter is doing this.

Finally did a succesful test on a physical Pixel 5.

Closes #186
(All workarounds can also be removed)