Highlight deprecated commands using title style

Highlight deprecated commands using title style

After Andrew's latest patch, I noticed that the deprecation warnings
could use the (so-called) title style when printing command names.
This patch implements this idea.

gdb/ChangeLog
2020-12-15  Tom Tromey  <tromey@adacore.com>

	* cli/cli-decode.c (deprecated_cmd_warning): Use title style for
	command names.

gdb/testsuite/ChangeLog
2020-12-15  Tom Tromey  <tromey@adacore.com>

	* gdb.base/style.exp: Add deprecation tests.