Trac: Mention that new Gutenberg issues should be created on GitHub, until 5.0

[birgire]

Some Gutenberg tickets are created here:

​https://core.trac.wordpress.org/newticket

but they are usually closed and referred to the GitHub issue tracker:

​https://github.com/WordPress/gutenberg/issues

I was wondering if we should mention it, in few words, above the core trac form.

At least while Gutenberg has not merged into core yet.

Here's an example of such closed trac ticket:

​https://core.trac.wordpress.org/ticket/44885

[dd32]

I see no reason not to do this, we can revert the following change when Gutenberg merging begins and the issue tracking has been migrated to Trac.

[dd32]

In 7657:

Trac: Core: Add a mention of Gutenberg on the new ticket form, to redirect bug reporters to the GitHub issue tracker.

H/t birgire.
Fixes #3797.