flatpak: Upgrade platform to 21.08

We need Rust with the 2021 edition.

Author: YaLTeR

build-aux/flatpak/rs.bxt.BunnymodXT.json

@@ -1,7 +1,7 @@
 {
     "app-id": "rs.bxt.BunnymodXT",
     "runtime": "org.freedesktop.Sdk",
-    "runtime-version": "20.08",
+    "runtime-version": "21.08",
     "command": "/usr/bin/true",
     "sdk": "org.freedesktop.Sdk",
     "sdk-extensions": [