Align Deno yargs_parser version with package.json version.

Refs #2136 and
https://github.com/yargs/yargs/pull/2231/files where it looks like the
Deno shim got missed in the version bump.

This should allow `.env()` to work in Deno resolving the original

lib/platform-shims/deno.ts

@@ -13,7 +13,7 @@ import {
 
 import cliui from 'https://deno.land/x/cliui@v7.0.4-deno/deno.ts';
 import escalade from 'https://deno.land/x/escalade@v3.0.3/sync.ts';
-import Parser from 'https://deno.land/x/yargs_parser@v20.2.4-deno/deno.ts';
+import Parser from 'https://deno.land/x/yargs_parser@v21.1.1-deno/deno.ts';
 import y18n from 'https://deno.land/x/y18n@v5.0.0-deno/deno.ts';
 import {YError} from '../../build/lib/yerror.js';