When we specify not Oracle JDK but OpenJDK for --jdk option, make/build.sh fails.

$ ~/jdk-15.0.1+9/bin/java -version
openjdk version "15.0.1" 2020-10-20
OpenJDK Runtime Environment AdoptOpenJDK (build 15.0.1+9)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 15.0.1+9, mixed mode, sharing)

$ bash make/build.sh --jdk ~/jdk-15.0.1+9
[build.sh][ERROR] JDK 8 or newer is required to build jtreg

This is caused by a difference in the output of java -version command on OpenJDK and Oracle JDK.

$ java -version
java version "1.8.0_281"
Java(TM) SE Runtime Environment (build 1.8.0_281-b09)
Java HotSpot(TM) 64-Bit Server VM (build 25.281-b09, mixed mode)