Change webchat.kde.org links to go.kde.org/matrix links

This changes the webchat.kde.org links we used to use with go.kde.org/matrix, which is our hosted matrix.to redirector. The main benefit is that it's way easier to see what you're jumping into, including a choice of client - instead of users being dumped into Element unknowingly:

(NeoChat is default here because that is my preferred client, otherwise it will give you a choice and webchat.kde.org is still the default Element)