(maint) Bump Ruby in static analysis workflow

Bump Ruby version in the static code analysis workflow to 2.6, as 2.5
will be EOL soon.

Author: GabrielNagy

.github/workflows/static_code_analysis.yaml

@@ -12,7 +12,7 @@ jobs:
     name: Run checks
 
     env:
-      ruby_version: 2.5
+      ruby_version: 2.6
       extra_checks: check:symlinks check:git_ignore check:dot_underscore check:test_file
 
     runs-on: 'ubuntu-18.04'