Drop support for Ruby <2.6 (#2027)

Ruby 2.5 reached EOL with the 2.5.9 release on 2021-04-05, we're now
starting to see many deprecations of other libraries and so it's time
for us to drop it too.

http://www.ruby-lang.org/en/news/2021/04/05/ruby-2-5-9-released/

.circleci/config.yml

@@ -45,21 +45,6 @@ default_job: &default_job
     - run: bundle exec appraisal rake
 
 jobs:
-  ruby-25:
-    <<: *default_job
-    docker:
-      - image: circleci/ruby:2.5.0-node-browsers
-        environment:
-          PGHOST: localhost
-          PGUSER: administrate
-          RAILS_ENV: test
-          BUNDLER_VERSION: 2.1.4
-      - image: postgres:10.1-alpine
-        environment:
-          POSTGRES_USER: administrate
-          POSTGRES_DB: ruby25
-          POSTGRES_PASSWORD: ""
-
   ruby-26:
     <<: *default_job
     docker:
@@ -94,4 +79,3 @@ workflows:
     jobs:
       - ruby-27
       - ruby-26
-      - ruby-25

Gemfile

@@ -4,17 +4,14 @@ gemspec
 
 gem "administrate-field-image"
 gem "faker"
+gem "front_matter_parser"
 gem "globalid"
 gem "kaminari-i18n"
 gem "pg"
 gem "redcarpet"
 gem "sentry-raven"
 gem "unicorn"
 
-# Version 1.0.1 invokes `YAML.safe_load` with arguments that are incompatible
-# with Ruby 2.5, causing an `ArgumentError: unknown keyword: permitted_classes`
-gem "front_matter_parser", "1.0.0"
-
 group :development, :test do
   gem "appraisal"
   gem "awesome_print"

Gemfile.lock

@@ -330,7 +330,7 @@ DEPENDENCIES
   factory_bot_rails
   faker
   formulaic
-  front_matter_parser (= 1.0.0)
+  front_matter_parser
   globalid
   i18n-tasks (= 0.9.34)
   kaminari-i18n

docs/getting_started.md

@@ -3,7 +3,7 @@ title: Getting Started
 ---
 
 Administrate is released as a Ruby gem, and can be installed on Rails
-applications version 5.0 or greater. We support Ruby 2.4 and up.
+applications version 5.0 or greater. We support Ruby 2.6 and up.
 
 First, add the following to your Gemfile:
 

gemfiles/rails50.gemfile

@@ -4,13 +4,13 @@ source "https://rubygems.org"
 
 gem "administrate-field-image"
 gem "faker"
+gem "front_matter_parser"
 gem "globalid"
 gem "kaminari-i18n"
 gem "pg", "0.21.0"
 gem "redcarpet"
 gem "sentry-raven"
 gem "unicorn"
-gem "front_matter_parser", "1.0.0"
 gem "actionpack", "~> 5.0.0"
 gem "actionview", "~> 5.0.0"
 gem "activerecord", "~> 5.0.0"

gemfiles/rails51.gemfile

@@ -4,13 +4,13 @@ source "https://rubygems.org"
 
 gem "administrate-field-image"
 gem "faker"
+gem "front_matter_parser"
 gem "globalid"
 gem "kaminari-i18n"
 gem "pg", "0.21.0"
 gem "redcarpet"
 gem "sentry-raven"
 gem "unicorn"
-gem "front_matter_parser", "1.0.0"
 gem "actionpack", "~> 5.1.0"
 gem "actionview", "~> 5.1.0"
 gem "activerecord", "~> 5.1.0"

gemfiles/rails52.gemfile

@@ -4,13 +4,13 @@ source "https://rubygems.org"
 
 gem "administrate-field-image"
 gem "faker"
+gem "front_matter_parser"
 gem "globalid"
 gem "kaminari-i18n"
 gem "pg"
 gem "redcarpet"
 gem "sentry-raven"
 gem "unicorn"
-gem "front_matter_parser", "1.0.0"
 gem "actionpack", "~> 5.2.0"
 gem "actionview", "~> 5.2.0"
 gem "activerecord", "~> 5.2.0"

gemfiles/rails60.gemfile

@@ -4,13 +4,13 @@ source "https://rubygems.org"
 
 gem "administrate-field-image"
 gem "faker"
+gem "front_matter_parser"
 gem "globalid"
 gem "kaminari-i18n"
 gem "pg"
 gem "redcarpet"
 gem "sentry-raven"
 gem "unicorn"
-gem "front_matter_parser", "1.0.0"
 gem "rails", "~> 6.0.3.4"
 
 group :development, :test do

gemfiles/rails61.gemfile

@@ -4,13 +4,13 @@ source "https://rubygems.org"
 
 gem "administrate-field-image"
 gem "faker"
+gem "front_matter_parser"
 gem "globalid"
 gem "kaminari-i18n"
 gem "pg"
 gem "redcarpet"
 gem "sentry-raven"
 gem "unicorn"
-gem "front_matter_parser", "1.0.0"
 gem "rails", "~> 6.1"
 
 group :development, :test do