Support for Java 11

[jeffawx]

MacOS, installed with brew install protop, v0.9.0

Run protop init, get error:

Error: LinkageError occurred while loading main class io.protop.cli.ProtopCli
	java.lang.UnsupportedClassVersionError: io/protop/cli/ProtopCli has been compiled by a more recent version of the Java Runtime (class file version 57.0), this version of the Java Runtime only recognizes class file versions up to 55.0

Java version:

openjdk version "11.0.11" 2021-04-20
OpenJDK Runtime Environment AdoptOpenJDK-11.0.11+9 (build 11.0.11+9)
OpenJDK 64-Bit Server VM AdoptOpenJDK-11.0.11+9 (build 11.0.11+9, mixed mode)

Expected behavior:
Given Java 11 is still widely used, better to support Java 11