Bump required_ruby_version to 2.6

Author: agrare

.github/workflows/ci.yaml

@@ -11,7 +11,6 @@ jobs:
     strategy:
       matrix:
         ruby-version:
-        - '2.5'
         - '2.6'
         - '2.7'
         - '3.0'

linux_admin.gemspec

@@ -26,7 +26,7 @@ registration, updates, etc.
   spec.executables   = `git ls-files -- bin/*`.split("\n")
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = ">= 2.0.0"
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.6")
 
   spec.add_development_dependency "manageiq-style"
   spec.add_development_dependency "rake"