Fix version check for Java 17

* Fixes https://github.com/rbenv/ruby-build/issues/1798
* OpenJDK 17 reports:
  $ java -version
  openjdk version "17" 2021-09-14
  There is no dot in the version, so we need to only use the first match from grep -o.
* Clarify it is a minimum required java version, not an exact version.