Fix gemspec and readme, Devise v5 will support Rails >= 7, not 6

As per our test matrix.

Gemfile.lock

@@ -14,7 +14,7 @@ PATH
     devise (5.0.0.beta)
       bcrypt (~> 3.0)
       orm_adapter (~> 0.1)
-      railties (>= 6.0.0)
+      railties (>= 7.0)
       responders
       warden (~> 1.2.3)
 
@@ -309,4 +309,4 @@ DEPENDENCIES
   webrat
 
 BUNDLED WITH
-   4.0.3
+  4.0.3

README.md

@@ -137,17 +137,17 @@ Please note that the command output will show the variable value being used.
 #### BUNDLE_GEMFILE
 We can use this variable to tell bundler what Gemfile it should use (instead of the one in the current directory).
 Inside the [gemfiles](https://github.com/heartcombo/devise/tree/main/gemfiles) directory, we have one for each version of Rails we support. When you send us a pull request, it may happen that the test suite breaks using some of them. If that's the case, you can simulate the same environment using the `BUNDLE_GEMFILE` variable.
-For example, if the tests broke using Ruby 3.0.0 and Rails 6.0, you can do the following:
+For example, if the tests broke using Ruby 3.4 and Rails 8.0, you can do the following:
 ```bash
-rbenv shell 3.0.0 # or rvm use 3.0.0
-BUNDLE_GEMFILE=gemfiles/Gemfile-rails-6-0 bundle install
-BUNDLE_GEMFILE=gemfiles/Gemfile-rails-6-0 bin/test
+chruby 3.4.0 # or rbenv shell 3.4.0, or rvm use 3.4.0, etc.
+BUNDLE_GEMFILE=gemfiles/Gemfile-rails-8-0 bundle install
+BUNDLE_GEMFILE=gemfiles/Gemfile-rails-8-0 bin/test
 ```
 
 You can also combine both of them if the tests broke for Mongoid:
 ```bash
-BUNDLE_GEMFILE=gemfiles/Gemfile-rails-6-0 bundle install
-BUNDLE_GEMFILE=gemfiles/Gemfile-rails-6-0 DEVISE_ORM=mongoid bin/test
+BUNDLE_GEMFILE=gemfiles/Gemfile-rails-8-0 bundle install
+BUNDLE_GEMFILE=gemfiles/Gemfile-rails-8-0 DEVISE_ORM=mongoid bin/test
 ```
 
 ### Running tests
@@ -181,7 +181,7 @@ Once you have solidified your understanding of Rails and authentication mechanis
 
 ## Getting started
 
-Devise 4.0 works with Rails 6.0 onwards. Run:
+Devise 5 works with Rails 7 onwards. Run:
 
 ```sh
 bundle add devise

devise.gemspec

@@ -30,6 +30,6 @@ Gem::Specification.new do |s|
   s.add_dependency("warden", "~> 1.2.3")
   s.add_dependency("orm_adapter", "~> 0.1")
   s.add_dependency("bcrypt", "~> 3.0")
-  s.add_dependency("railties", ">= 6.0.0")
+  s.add_dependency("railties", ">= 7.0")
   s.add_dependency("responders")
 end