Set the required Ruby version to 3.2.0

I'll relax this later

.rubocop.yml

@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.6
+  TargetRubyVersion: 3.2
 
 Style/StringLiterals:
   Enabled: true

.tool-versions

@@ -0,0 +1 @@
+ruby 3.2.0

nostr.gemspec

@@ -12,7 +12,7 @@ Gem::Specification.new do |spec|
   spec.description = "Client and relay implementation of the Nostr protocol."
   spec.homepage = "https://github.com/wilsonsilva/nostr"
   spec.license = "MIT"
-  spec.required_ruby_version = ">= 2.6.0"
+  spec.required_ruby_version = ">= 3.2.0"
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = "https://github.com/wilsonsilva/nostr"