[11u] Desupport building with Java 10?

[Lindenmaier, Goetz]

Hi,

OpenJDK has the policy that each jdk should be able to
be bootstrapped with the previous Java version.
See also DEFAULT_ACCEPTABLE_BOOT_VERSIONS [1].
This makes sense for new Java versions but not for
updates of LTSes.  For jdk11u, using a Java 10 jdk
for bootstrapping even is a security issue as there
is no maintained jdk10.

On the other side we had to fix a few changes in
the past to work with Java 10.  We should avoid this
effort.

I propose to give up supporting Java 10 in the build
in 11.0.16 and to change DEFAULT_ACCEPTABLE_BOOT_VERSIONS
accordingly.

If there are no objections I'll do the corresponding
change for 11.0.16.

Best regards,
  Goetz.

[1] https://github.com/openjdk/jdk11u-dev/blob/master/make/autoconf/version-numbers