Upgrade version and docs to 7.0 (#528)

## All PRs:

* [x] Has tests
* [x] Documentation updated

## Adding a new header

N/A

## Adding a new CSP directive

N/A

Closes https://github.com/github/secure_headers/issues/480

docs/upgrading-to-7-0.md

@@ -0,0 +1,12 @@
+## X-Xss-Protection is set to 0 by default
+
+Version 6 and below of `secure_headers` set the `X-Xss-Protection` to `1; mode=block` by default. This was done to protect against reflected XSS attacks. However, this header is no longer recommended (see https://github.com/github/secure_headers/issues/439 for more information).
+
+If any functionality in your app depended on this header being set to the previous value, you will need to set it explicitly in your configuration.
+
+```ruby
+# config/initializers/secure_headers.rb
+SecureHeaders::Configuration.default do |config|
+  config.x_xss_protection = "1; mode=block"
+end
+```

lib/secure_headers/version.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module SecureHeaders
-  VERSION = "6.5.0"
+  VERSION = "7.0.0"
 end

secure_headers.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |gem|
   gem.description   = 'Add easily configured security headers to responses
     including content-security-policy, x-frame-options,
     strict-transport-security, etc.'
-  gem.homepage      = "https://github.com/twitter/secureheaders"
+  gem.homepage      = "https://github.com/github/secure_headers"
   gem.license       = "MIT"
   gem.files         = `git ls-files`.split($INPUT_RECORD_SEPARATOR)
   gem.executables   = gem.files.grep(%r{^bin/}).map { |f| File.basename(f) }