Build: Remove all sourceMappingURL comments when copying Gutenberg files

[adamsilverstein]

Summary

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).

Test plan

• Verify the verify:source-maps build check passes with this change when the vips package is included in the Gutenberg build output
• Confirm existing source map removal still works correctly for all other packages

🤖 Generated with Claude Code

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[ellatrix]

Tried it in #10968 and it fixes the issue.