Add a deprecation warning for using Scala 2.12.4 with Bloop

[Gedochao]

Closes #1382
Adds a deprecation warning for when Scala 2.12.4 is used with Bloop (which can be suppressed as any other deprecation warning), as per #1382 (comment)

scala-cli -e 'println("Hello")' -S 2.12.4 --jvm 8
# [warn] Scala 2.12.4 has been deprecated for use with Bloop.
# [warn] It may lead to infinite compilation.
# [warn] To disable the build server, pass --server=false.
# [warn] Refer to https://github.com/VirtusLab/scala-cli/issues/1382 and https://github.com/sbt/zinc/issues/1010
# Compiling project (Scala 2.12.4, JVM (8))
# Non-compiled module 'compiler-bridge_2.12' for Scala 2.12.4. Compiling...
# Error compiling project (Scala 2.12.4, JVM (8))
# Compilation failed 

scala-cli -e 'println("Hello")' -S 2.12.4 --jvm 8 --server=false
# Hello

scala-cli -e 'println("Hello")' -S 2.12.4 --jvm 8 --suppress-deprecated-warning
# Compiling project (Scala 2.12.4, JVM (8))
# Non-compiled module 'compiler-bridge_2.12' for Scala 2.12.4. Compiling...
# Error compiling project (Scala 2.12.4, JVM (8))
# Compilation failed

[tgodzik]

Is the issue connected to the sbt bridge? I think I've seen it trying to compile with newer JDK in sbt also.

[Gedochao]

Is the issue connected to the sbt bridge? I think I've seen it trying to compile with newer JDK in sbt also.

@tgodzik The SBT bridge should compile, truth be told...
Even if the compiler bridge succeeded, the compilation would then hang on Zinc, and that won't be fixed:

• Throw an error message when Scala 2.12.4 is used sbt/zinc#1010

2.12.4 also does work with --server=false, if someone is truly desperate for this to work.