Build/Test Tools: Remove the requirement to clone/build Gutenberg.

This iterates on the changes from [61438] by removing the need to:
- Check out the WordPress/gutenberg repository at the pinned hash.
- Run `npm install` within that checkout.
- Run `npm build` within that checkout.

Instead, the build script will now download a prebuilt zip file published to the GitHub Container Registry by a GitHub Actions workflow recently merged to the Gutenberg Repository (related PR: https://github.com/WordPress/gutenberg/pull/75844).

This also removes redundant code responsible for:
- Copying files from the `gutenberg` directory to the appropriate locations during the build script in favor of using `grunt copy`.
- Modifying built files to replace specific text, such as `sourceMappingURL`, in favor of `grunt replace`.

The remaining files within the `tools/gutenberg` directory have been renamed to remove `gutenberg` from the file names. Since these are already nested in a `gutenberg` directory, that was redundant.

Since the intention of the pinned value for the repository in the `package.json` file is to specify a full-length commit hash, `ref` has been renamed to `sha`. In Git `ref` encompasses branches, tags, and commit hashes, so this hopefully makes it more clear that something like `branch-name` should not be used.

Follow up to [61438], [61439], [61458], [61492], [61677], [61867].

Props desrosj, dmsnell, westonruter, mcsf, jorbin.
See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@61873 602fd350-edb4-49c9-b593-d223f7449a82