Use --release flag in JDK 9+ based builds

[lukaseder]

Until jOOQ 3.11, jOOQ was built using JDK 8. Cross releasing to JDK 6 & 8 was implemented using

• A home grown preprocessor
• Different maven source and target versions
• The animal-sniffer plugin

Details here:
https://blog.jooq.org/2016/03/01/how-to-support-java-6-8-9-in-a-single-api/

With jOOQ 3.12, the build is based on JDK 10 (or 11, depending on the release timeline). We can now use the new --release flag from the JDK's compiler, see JEP 247:

• http://openjdk.java.net/jeps/247
• https://stackoverflow.com/q/43102787/521799

This will be a bit more robust than using the animal-sniffer plugin.