Refactor minify-assets.sh script

What does this MR do and why?

Simplify the minify-assets.sh script so it's easier to understand and maintain.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

2. Check out branch.

3. Run make clean && make compile.

4. Run docker build -f dockerfiles/single.Dockerfile . --build-arg VER=15.10. It should succeed.

5. Update minify-assets.sh to make an invalid call. For example:

   -if $MINIFY_BIN "${MINIFY_FLAGS[@]}" --type=html --match="\.html$" -o "${TARGET}/${VER}/" "${TARGET}/${VER}"; then
   +if $MINIFY_BIN "${MINIFY_FLAGS[@]}" --type=html --matches="\.html$" -o "${TARGET}/${VER}/" "${TARGET}/${VER}"; then

6. Run docker build -f dockerfiles/single.Dockerfile . --build-arg VER=15.10. It should fail.

7. Run cd public.

8. Run ../scripts/minify-assets.sh ./ ./. It should succeed.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.