Minimum is now 2.6

Getting an error with 2.5 on CI

```
Your RubyGems version (2.7.6.3) has a bug that prevents `required_ruby_version` from working for Bundler. Any scripts that use `gem install bundler` will break as soon as Bundler drops support for your Ruby version. Please upgrade RubyGems to avoid future breakage and silence this warning by running `gem update --system 3.2.3`
```

It's easier to rev the version than to work around the problem

.github/workflows/ci.yml

@@ -11,7 +11,6 @@ jobs:
       fail-fast: false
       matrix:
         ruby:
-          - 2.5
           - 2.6
           - 2.7
           - 3.0

.standard.yml

@@ -1 +1 @@
-ruby_version: 2.5
+ruby_version: 2.6

CHANGELOG.md

@@ -1,6 +1,6 @@
 ## HEAD (unreleased)
 
-- Minimum Ruby version is now Ruby 2.5
+- Minimum Ruby version is now Ruby 2.6
 - Add `bigdecimal` as a dependency.
 
 ## 0.2.7

get_process_mem.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |gem|
   gem.executables = gem.files.grep(%r{^bin/}).map { |f| File.basename(f) }
   gem.require_paths = ["lib"]
 
-  gem.required_ruby_version = ">= 2.5"
+  gem.required_ruby_version = ">= 2.6"
 
   gem.add_dependency "ffi", "~> 1.0"
   gem.add_dependency "bigdecimal", ">= 2.0"