Refactor Snippets::UpdateService to allow multiple files

What does this MR do?

In !32649 (merged) we introduced a new param snippet_file into the snippets create service. In this MR, we update the Snippets::UpdateService to use that param as well if present.

Refs #217733 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [ ] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content