Migrate to slash command syntax

[ximon18]

In our code we currently do:

        .configure(|c| c.prefix("!")) // set the bot's prefix to "!"

As this was the norm at the time the bot was written.

However, https://discord.com/blog/welcome-to-the-new-era-of-discord-apps says:

Secondly, we’re also standardizing how app commands work so they’re easier and safer to use. In a nutshell, the ‘old way’ was talking to a bot (e.g. sending a message, which it would read) to tell it what to do. The ‘new way’ is to type a slash “/” to bring up a menu of available commands in the server, so you can select the one you want. These are called Slash Commands. We love using them, and we know millions of you who are using them already do, too!

And more importantly:

September 1, 2022 is the final switchover date. This means that, if the developer of one of the apps you’re using has not made the switch yet, parts of the app might not work after that time.

So potentially we have until then to make any necessary updates to the bot code. But perhaps it's as simple as a one character change in our case?