Correct CTRL+U underline function in message compose

[tecnica3]

For several versions, Thunderbird does not use the CTRL+U function to underline texts.
The same shortcut has been assigned to the search function.
In other words, there are two functions assigned to the same shortcut and no one ever noticed?

[wsm]

@tecnica3, are you still seeing this issue?

I am not currently seeing this in Thunderbird version 145, daily builds.