Update playground link's edition to 2021

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

Author: JohnTitor

app/components/crate-sidebar.js

@@ -31,7 +31,7 @@ export default class CrateSidebar extends Component {
     let playgroundCrate = playgroundCrates.find(it => it.name === this.args.crate.name);
     if (!playgroundCrate) return;
 
-    return `https://play.rust-lang.org/?edition=2018&code=use%20${playgroundCrate.id}%3B%0A%0Afn%20main()%20%7B%0A%20%20%20%20%2F%2F%20try%20using%20the%20%60${playgroundCrate.id}%60%20crate%20here%0A%7D`;
+    return `https://play.rust-lang.org/?edition=2021&code=use%20${playgroundCrate.id}%3B%0A%0Afn%20main()%20%7B%0A%20%20%20%20%2F%2F%20try%20using%20the%20%60${playgroundCrate.id}%60%20crate%20here%0A%7D`;
   }
 
   get canHover() {

tests/components/crate-sidebar/playground-button-test.js

@@ -48,7 +48,7 @@ module('Component | CrateSidebar | Playground Button', function (hooks) {
     this.version = (await this.crate.versions).firstObject;
 
     let expectedHref =
-      'https://play.rust-lang.org/?edition=2018&code=use%20aho_corasick%3B%0A%0Afn%20main()%20%7B%0A%20%20%20%20%2F%2F%20try%20using%20the%20%60aho_corasick%60%20crate%20here%0A%7D';
+      'https://play.rust-lang.org/?edition=2021&code=use%20aho_corasick%3B%0A%0Afn%20main()%20%7B%0A%20%20%20%20%2F%2F%20try%20using%20the%20%60aho_corasick%60%20crate%20here%0A%7D';
 
     await render(hbs`<CrateSidebar @crate={{this.crate}} @version={{this.version}} />`);
     assert.dom('[data-test-playground-button]').hasAttribute('href', expectedHref);