Remove unnecessary isolation_level setting from load_defaults 7.0

The default value of `ActiveSupport::IsolatedExecutionState.isolation_level` is set to `:thread`, regardless of whether `load_defaults 7.0` is used or not.

- https://github.com/rails/rails/blob/v7.0.4/activesupport/lib/active_support/isolated_execution_state.rb

If so, there is no need to set this in `load_defaults 7.0`. This Pull Request has been created to remove this unnecessary setting

Author: r7kamura

guides/source/configuring.md

@@ -106,7 +106,6 @@ Below are the default values associated with each target version. In cases of co
 - [`config.active_support.cache_format_version`](#config-active-support-cache-format-version): `7.0`
 - [`config.active_support.executor_around_test_case`](#config-active-support-executor-around-test-case): `true`
 - [`config.active_support.hash_digest_class`](#config-active-support-hash-digest-class): `OpenSSL::Digest::SHA256`
-- [`config.active_support.isolation_level`](#config-active-support-isolation-level): `:thread`
 - [`config.active_support.key_generator_hash_digest_class`](#config-active-support-key-generator-hash-digest-class): `OpenSSL::Digest::SHA256`
 
 #### Default Values for Target Version 6.1

railties/lib/rails/application/configuration.rb

@@ -239,7 +239,6 @@ def load_defaults(target_version)
             active_support.key_generator_hash_digest_class = OpenSSL::Digest::SHA256
             active_support.cache_format_version = 7.0
             active_support.executor_around_test_case = true
-            active_support.isolation_level = :thread
           end
 
           if respond_to?(:action_mailer)