This PR improves the regular expression used for matching the java
version string, which varies a lot depending on the java vendor and
version.

These are the example strings we now test for:

java version "1.7.0_80"
openjdk version "11.0.1" 2018-10-16
openjdk version "11.0.1" 2018-10-16
java version "1.6.0_36"
openjdk version "1.8.0_192"
openjdk 11.0.11 2021-04-20 LTS

The last one is a new test added on behalf of #6081, which is
still broken on today's CentOS 7 default JDK package.

openjdk 11.0.11 2021-04-20 LTS
OpenJDK Runtime Environment 18.9 (build 11.0.11+9-LTS)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.11+9-LTS, mixed mode, sharing)

==> Evaluation "21c6caf7" finished with status "complete" but failed to place all allocations:
    Task Group "example" (failed to place 1 allocation):
      * Constraint "${driver.java.version} >= 11.0.0": 1 nodes excluded by filter
    Evaluation "2b737d48" waiting for additional capacity to place r...

Restore 0.8.x behavior where java driver is marked as detected when
`java -version` exits with 0 but returns unexpected output.

Furthermore, we restore behavior when `java -version` where we parse the
first three lines of `java -version` but ignore rest.

If `java -version` returns less than 3 lines, Nomad 0.8.7 would panic.
In this implementation, we'd still mark java as detected but returns
empty version.

The 0.8.7 logic for detecting java version is found in
https://github.com/hashicorp/nomad/blob/v0.8.7/client/driver/java.go#L132-L172
.

I punt on revamping how we can be more resilient to java -version
syntax, and aimed for preserving existing behavior instead.

ignore _JAVA_OPTIONS line in `java -version`, as it's relevant.