Meta/3.19.0 beta.1 (#3220)

* Bump version to 3.19.0 in nuspec file

* Update app footer year and version in docfx.json

* Bump version prefix to 3.19.0

* Update CHANGELOG for version 3.19.0-beta.1

docs/docfx.json

@@ -59,7 +59,7 @@
     "globalMetadata": {
       "_appTitle": "Discord.Net Documentation",
       "_appName": "Discord.Net",
-      "_appFooter": "Discord.Net © 2015-2025 3.18.0",
+      "_appFooter": "Discord.Net © 2015-2026 3.19.0",
       "_enableSearch": true,
       "_appLogoPath": "marketing/logo/SVG/Logomark Purple.svg",
       "_appFaviconPath": "favicon.png"