Replace mention of IRC with Discord

[Shadlock0133]

There are still few mentions of IRC in the repo. Removing them is not a priority but it will avoid confusion for newcomers.

[initram]

Should these places be replaced with references to the rust discord server, or just be removed outright?

[Shadlock0133]

Probably replaced. Gonna update title.

[radu-matei]

Opened #61531 to address this.

[llebout]

This is horrible. Replacing IRC with Discord.. :/

[Shadlock0133]

@Leo-LB If you haven't read about it yet, here is the post from April.

[llebout]

@Shadlock0133 Yeah I read already. I'd remove the mention of any particular discussion channel, instead of putting Discord in. I can't wait for Matrix to become more mature so the Rust project can use it. Falling back to Discord is quite bad for a FOSS project.

[Shadlock0133]

@Leo-LB IMO we should just mirror subreddit solution and point to Discord.