Drop Ruby 2.5 support

Because it has reached EOL for more than 1.5 years and it won't be
supported by the next reline version either.

lib/irb.rb

@@ -598,11 +598,7 @@ module IRB
 
       if exc.backtrace
         order = nil
-        if '2.5.0' == RUBY_VERSION
-          # Exception#full_message doesn't have keyword arguments.
-          message = exc.full_message # the same of (highlight: true, order: bottom)
-          order = :bottom
-        elsif '2.5.1' <= RUBY_VERSION && RUBY_VERSION < '3.0.0'
+        if RUBY_VERSION < '3.0.0'
           if STDOUT.tty?
             message = exc.full_message(order: :bottom)
             order = :bottom

lib/irb/irb.gemspec

@@ -34,7 +34,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.5")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.6")
 
   spec.add_dependency "reline", ">= 0.3.0"
 end

test/irb/test_context.rb

@@ -492,7 +492,7 @@ module TestIRB
         irb.eval_input
       end
       assert_empty err
-      if '2.5.0' <= RUBY_VERSION && RUBY_VERSION < '3.0.0' && STDOUT.tty?
+      if RUBY_VERSION < '3.0.0' && STDOUT.tty?
         expected = [
           :*, /Traceback \(most recent call last\):\n/,
           :*, /\t 2: from \(irb\):1:in `<main>'\n/,
@@ -523,7 +523,7 @@ module TestIRB
         irb.eval_input
       end
       assert_empty err
-      if '2.5.0' <= RUBY_VERSION && RUBY_VERSION < '3.0.0' && STDOUT.tty?
+      if RUBY_VERSION < '3.0.0' && STDOUT.tty?
         expected = [
           :*, /Traceback \(most recent call last\):\n/,
           :*, /\t 2: from \(irb\):1:in `<main>'\n/,
@@ -560,7 +560,7 @@ module TestIRB
         irb.eval_input
       end
       assert_empty err
-      if '2.5.0' <= RUBY_VERSION && RUBY_VERSION < '3.0.0' && STDOUT.tty?
+      if RUBY_VERSION < '3.0.0' && STDOUT.tty?
         expected = [
           :*, /Traceback \(most recent call last\):\n/,
           :*, /\t... \d+ levels...\n/,

test/irb/test_ruby_lex.rb

@@ -181,9 +181,6 @@ module TestIRB
     end
 
     def test_endless_range_at_end_of_line
-      if Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.6.0')
-        pend 'Endless range is available in 2.6.0 or later'
-      end
       input_with_prompt = [
         PromptRow.new('001:0: :> ', %q(a = 3..)),
         PromptRow.new('002:0: :* ', %q()),