Ignore solargraph config file

What does this MR do?

Allows VSCode users (and some others) to use the Ruby Solargraph extension and the Solargraph gem to aid in development.

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