Unescaped left braces in regular expressions will be fatal in 5.30.

Unescaped left braces in regular expressions will be fatal in 5.30.

In 5.26, some uses of unescaped left braces were made fatal; they have
given a deprecation warning since 5.20. Due to an oversight, some cases
were missed, and did not give a deprecation warning. They do now.

This patch changes said deprecation warning to mention the Perl version
in which the use of an unescaped left brace will be fatal (5.30).

The patch also cleans up some unnecessary quotes inside a C<> construct
in the discussion of this warning in perldiag.pod.