Make Java 17 the minimum runtime and build with --release 17

Connector/J 4.0 targets Java 17 as the minimum runtime.
The build currently compiles the baseline with -source/-target 1.8 and layers three multi-release trees on top (java9 for module-info, java11 for SocketHelper, java16 for UnixDomainSocket).

• Compile everything with <release>17</release> and drop the java8 and java11 Maven profiles and the -java8 dependency variants (logback-java8.version).
• Fold src/main/java9, java11 and java16 back into src/main/java, and remove the Multi-Release manifest entry and the multiReleaseOutput compiler executions.
• Remove the runtime Java version checks (the placeholder UnixDomainSocket, getJavaVersion() helpers in tests).
• State the new minimum in the README, the changelog and the _java_version connection attribute documentation.