irc: Use strikethrough style rather than grey color for closed tasks

Enough folks have had trouble reading grey foreground text to make it reasonable to change. We will try using strikethrough style instead. This styling will hopefully be more intuitive as it matches the styling used by Phabricator itself for closed task titles.

strikethrough is not supported by all IRC clients, but we hope that those clients that will not render in a struck font will still render the text in a reasonable way.

Bug: T360353