Build: Remove all sourceMappingURL comments when copying Gutenberg files.

The `removeSourceMaps` regex in `copy-gutenberg-build.js` was missing the `/g` (global) flag, so it only stripped the **first** `//# sourceMappingURL=` comment per file.

Bundled files such as the `@wordpress/vips` web worker can contain **multiple** `sourceMappingURL` references from concatenated modules (esbuild builds worker bundles with `sourcemap: true`, and when webpack bundles the module entry point it preserves comments from source modules). This causes the `verify:source-maps` build check to fail:

```
Warning: The build/wp-includes/js/dist/script-modules/vips/worker.js file must not contain a sourceMappingURL.
```

Adding the `/g` flag ensures every occurrence is removed, consistent with the existing `replace:source-maps` Grunt task which already uses the global flag.

Fixes the build failure reported in #10968 (comment).

Developed in #10970.

Props adamsilverstein.
See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@61677 602fd350-edb4-49c9-b593-d223f7449a82

Author: ellatrix

tools/gutenberg/copy-gutenberg-build.js

@@ -922,7 +922,7 @@ async function main() {
 
 	// Transform function to remove source map comments from all JS files
 	const removeSourceMaps = ( content ) => {
-		return content.replace( /\/\/# sourceMappingURL=.*$/m, '' ).trimEnd();
+		return content.replace( /\/\/# sourceMappingURL=.*$/gm, '' ).trimEnd();
 	};
 
 	if ( fs.existsSync( scriptsSrc ) ) {