Update solargraph requirement from ~> 0.55.1 to ~> 0.56.0 (#598)

Updates the requirements on [solargraph](https://github.com/castwide/solargraph) to permit the latest version.
- [Changelog](https://github.com/castwide/solargraph/blob/master/CHANGELOG.md)
- [Commits](castwide/solargraph@v0.55.1...v0.56.0)

---
updated-dependencies:
- dependency-name: solargraph
  dependency-version: 0.56.0
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile

@@ -12,7 +12,7 @@ group :development do
   gem "rake", "~> 13.3.0"
   gem "ruby-lsp", "~> 0.24.1"
   gem "semver2", "~> 3.4"
-  gem "solargraph", "~> 0.55.1"
+  gem "solargraph", "~> 0.56.0"
 end
 
 group :test do