databases/ruby-gdbm: Gemify gdbm

since gdbm is gemified in upstream after Ruby 2.5 [1]. Ruby 2.4 can also
use the gdbm gem because it is a gem!

• Add MOVED entry
• Bump PORTEPOCH because PORTVERSION goes barwards
• Update pkg-descr and WWW
• Remove unnecessary pkg-message

[1] https://bugs.ruby-lang.org/issues/5481

Update lang/ruby* ports:

• Do not build gdbm together with Ruby interpreter, but build as a rubygem
• Remove ${BUILD_WRKSRC}/ext/gdbm instead of moving it to ${WRKDIR}
• Update pkg-message to mention rubygem-gdbm instead of ruby-gdbm

Also following changes are made to ruby-gdbm consumers:

• Updade dependency to depend on rubygem-gdbm
• Bump PORTREVISION due to dependency change

PR: 230436
Submitted by: Yasuhiro KIMURA <yasu@utahime.org>
Sponsored by: HAW International, Inc.