New jtreg version numbers

[Jonathan Gibbons]

This is FYI for folk that use the OpenJDK Regression Test Harness, 
jtreg, to run OpenJDK tests.

For the past few years, jtreg has used a version numbering scheme "4.2 
bNN" where NN slowly incremented, instead of a more conventional 
"$MAJOR.$MINOR" numbering scheme. The reasons for this choice have long 
since disappeared, and so going forward, jtreg will use a 
"$MAJOR.$MINOR" numbering scheme, starting with 5.0.

Along with this change, jtreg will also require JDK 8 or later to 
compile and execute it.  That should not affect most people, for whom 
JDK 8 or later has long been recommended, although not required.  In the 
unlikely event that we need to fix an issue in a version of jtreg that 
can run on JDK 7, we can provide an update based on the latest 4.2 version.

For those folk that build jtreg from the tip of the repo, you may notice 
the related changes in the repo.

-- Jon