RFR: JDK-8189094: Change required boot jdk to JDK 9

[Martin Buchholz]

What's the canonical way to list all upgradeable modules?
There's a list in JEP 261, but naturally we can't consider it authoritative.

I was surprised that even java --describe-module doesn't tell me whether a
module is upgradeable.

I was surprised to see JEP 261 say that java.compiler is upgradeable, but
not jdk.compiler.
Perhaps thinking of the bootstrap use case?