migrate gitter links to discord

This follows up the announcement made on https://www.scala-lang.org/blog/2021/12/21/discord.html
that Discord is now the official Scala chat.

Author: ckipp01

README.md

@@ -1,7 +1,7 @@
 Dotty
 =====
 [![Dotty CI](https://github.com/lampepfl/dotty/workflows/Dotty/badge.svg?branch=master)](https://github.com/lampepfl/dotty/actions?query=branch%3Amaster)
-[![Join the chat at https://gitter.im/scala/scala](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/scala/scala?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Join the chat at https://discord.com/invite/scala](https://img.shields.io/discord/632150470000902164)](https://img.shields.io/discord/632150470000902164)
 
 * [Homepage](https://dotty.epfl.ch)
 * [Documentation](https://dotty.epfl.ch/docs)

project/Build.scala

@@ -329,7 +329,7 @@ object Build {
       "-project-logo", "docs/logo.svg",
       "-social-links:" +
         "github::https://github.com/lampepfl/dotty," +
-        "gitter::https://gitter.im/scala/scala," +
+        "discord::https://discord.com/invite/scala," +
         "twitter::https://twitter.com/scala_lang",
       // contains special definitions which are "transplanted" elsewhere
       // and which therefore confuse Scaladoc when accessed from this pkg

project/scripts/cmdScaladocTests

@@ -30,7 +30,7 @@ dist/target/pack/bin/scaladoc \
   "-skip-by-regex:.+\.internal($|\..+)" \
   "-skip-by-regex:.+\.impl($|\..+)" \
   -project-logo docs/logo.svg \
-  -social-links:github::https://github.com/lampepfl/dotty,gitter::https://gitter.im/scala/scala,twitter::https://twitter.com/scala_lang \
+  -social-links:github::https://github.com/lampepfl/dotty,discord::https://discord.com/invite/scala,twitter::https://twitter.com/scala_lang \
   -Ygenerate-inkuire \
   "-skip-by-id:scala.runtime.stdLibPatches" \
   "-skip-by-id:scala.runtime.MatchCase" \