Swap gitter references to Discord (#3486)

Gitter seems totally borked these days and most discussion is in
discord, this makes it official

.github/ISSUE_TEMPLATE.md

@@ -1,4 +1,4 @@
-Please don't open issues for questions, but ask in our gitter channel at https://gitter.im/lihaoyi/mill
+Please don't open issues for questions, but ask in our Discussions forum at https://github.com/com-lihaoyi/mill/discussions or Discord channel at https://discord.com/channels/632150470000902164/940067748103487558
 
 Mill installations via `coursier` or `cs` are unsupported.
 

CONTRIBUTING.adoc

@@ -1,6 +1,6 @@
 = Contributing to Mill
 :link-github: https://github.com/com-lihaoyi/mill
-:link-gitter: https://gitter.im/lihaoyi/mill
+:link-chat: https://discord.com/channels/632150470000902164/940067748103487558
 
 Thank you for considering contributing to Mill.
 
@@ -16,6 +16,6 @@ Here are some direct links:
 * {link-github}/discussions[Discussion Forum on GitHub] - A place to ask question and discuss all kind of questions around Mill
 * {link-github}/issues[Issue Tracker on GitHub] - Our issue tracker for bugs and features
 * {link-github}/pulls[Pull Requests on GitHub] - All new features and bug fixes find their way into Mill via a pull request. You can also sketch new ideas by creating a draft pull requests.
-{link-gitter}[Gitter Chat (Element)] - You can also join our chat room if you like more direct communication or to just say hello
+{link-chat}[Discord Chat] - You can also join our chat room if you like more direct communication or to just say hello
 
 To build docs locally, `mill docs.localPages`. The last line of the command tells you where to browse the generated pages. From there you can follow the breadcrumbs.

docs/supplemental-ui/partials/header-content.hbs

@@ -20,7 +20,7 @@
         <a class="navbar-item" href="{{{or site.url (or siteRootUrl siteRootPath)}}}/api/latest/index.html">API</a>
         <a class="navbar-item" href="https://github.com/com-lihaoyi/mill/issues">Issues</a>
         <a class="navbar-item" href="https://github.com/com-lihaoyi/mill/discussions">Discuss</a>
-        <a class="navbar-item" href="https://gitter.im/lihaoyi/mill">Chat</a>
+        <a class="navbar-item" href="https://discord.com/channels/632150470000902164/940067748103487558">Chat</a>
 
             <!--
         <a class="navbar-item" href="#">Home</a>

readme.adoc

@@ -2,7 +2,6 @@
 :idprefix:
 :idseparator: -
 :link-github: https://github.com/com-lihaoyi/mill
-:link-gitter: https://gitter.im/lihaoyi/mill
 :link-current-doc-site: https://mill-build.org
 :link-mill-moduledefs: https://github.com/com-lihaoyi/mill-moduledefs
 :example-scala-version: 3.3.4
@@ -20,7 +19,6 @@ ifdef::env-github[]
 endif::[]
 
 {link-github}/actions/workflows/actions.yml[image:{link-github}/actions/workflows/actions.yml/badge.svg[Build and Release]]
-{link-gitter}?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge[image:https://badges.gitter.im/Join%20Chat.svg[Gitter Chat]]
 https://index.scala-lang.org/com-lihaoyi/mill/mill-main[image:https://index.scala-lang.org/com-lihaoyi/mill/mill-main/latest.svg[Mill]]
 https://www.patreon.com/lihaoyi[image:https://img.shields.io/badge/patreon-sponsor-ff69b4.svg[Patreon]]