Update build.gradle

[Tulleb]

So we don't have this error anymore with more recent gradle versions:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':flutter_bcrypt:compileDebugKotlin'.
> Inconsistent JVM-target compatibility detected for tasks 'compileDebugJavaWithJavac' (1.8) and 'compileDebugKotlin' (17).

  Consider using JVM Toolchain: https://kotl.in/gradle/jvm/toolchain
  Learn more about JVM-target validation: https://kotl.in/gradle/jvm/target-validation 

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org.

BUILD FAILED in 13s
Error: Gradle task assembleInternalDebug failed with exit code 1