ruby-kramdown: Update to 2.3.0

Changelog:
kramdown 2.3.0 released

Although this is a minor version bump there is one breaking change:
Parsing of XML processing instructions was removed because they
are invalid for HTML5 documents.

This change should only affect a negligible amount of existing
kramdown documents since XML processing instructions were never
something a normal user would use.

Additionally, CVE-2020-14001 is addressed to avoid problems when
using the {::options /} extension together with the ���template���
option. This means updating is highly recommended!

Changes

    2 major changes:

New option ���forbidden_inline_options��� to restrict the
options allowed with the {::options /} extension. This also
addresses the security issue described in CVE-2020-14001.

Parsing of XML processing instructions is not done anymore
for kramdown documents because they are invalid for HTML5
(fixes issue #660 by Samuel Williams)

    1 minor change:

Several internal changes with respect to memory usage and
performance (PRs #654, #655, #665 by Ashwin Maroli)

    2 bug fixes:

Extend allowed characters in IDs set with headers to all
characters allowed by XML (fixes #658 by Samuel Williams)

Fix thread safety issue by moving global state into an
instance variable (fixes #663 by Samuel Williams)

    1 other change:

Documentation fixes and updates (issue #662 by Samuel
Williams, PR #656 by Noah Doersing)

(ryoon)