Bug 1825785: Only run WebGPU mochitests on mingw in mozilla-central. r=RyanVM

At the moment, WebGPU is only enabled in nightly builds. Bug 1750398 accidentally enabled the WebGPU mochitests in all ["release"](https://firefox-source-docs.mozilla.org/taskcluster/attributes.html#run-on-projects) builds, but that doesn't make sense yet.

Differential Revision: https://phabricator.services.mozilla.com/D174799

taskcluster/ci/test/mochitest.yml

@@ -974,7 +974,7 @@ mochitest-webgpu:
             default:
                 by-test-platform:
                     android.*: []
-                    .*mingw.*: ["release"]
+                    .*mingw.*: ["mozilla-central"]
                     .*shippable.*: ["mozilla-central"]
                     .*ccov.*: ['mozilla-central']
                     .*tsan.*: ['trunk']