Change Base's Bundle-RequireExecutionEnvironment to require Java8 (not 7)

To support new target platform (JBIDE-19776) which includes new Sapphire, here's a new proposal to update JBDS 9 / JBT 4.3 to require JDK 8 runtime.

Proposal support matrix:

• IDE installer will run wtih Java 6+ (but then user needs 8 to run it) [NO CHANGE]
• IDE will ONLY run on Java 8+ [NEW for JBDS 9]
• Server runtimes will run on existing JREs (from 4 - 9) [NO CHANGE]
• QE actually does not test on 4 or 5, only 6+
• Java 9 is "preview / smoketest" only, not supported yet

To force users to use a JDK 8 runtime to run Eclipse Mars / JBDS 9, we need to change the Base component's Bundle-RequireExecutionEnvironment to require Java8 (not 7).

Not sure how many plugins should be updated this way.