Fixing wrong link in 'Ruby on Rails Security Guide' [ci skip]

The URL

http://www.h-online.com/security/Symantec-reports-first-active-attack-on-a-DSL-router--/news/102352

points to an article titled 'The H is closing down'. The good one is:

http://www.h-online.com/security/news/item/Symantec-reports-first-active-attack-on-a-DSL-router-735883.html
Merge pull request #17716 from javiervidal/fixing_documentation

Fixing wrong link in 'Ruby on Rails Security Guide' [ci skip]
Remove outdated comments [ci skip]

They were introduced in 23b6def; the serial stuff has been removed since a5d80f8
Rename the primary key index when renaming a table in pg

Also checked to make sure this does not affect foreign key constraints.
(It doesn't).

Fixes #12856
Closes #14088
Fix build failures

For some reason changing `.find` to `.unscoped.find` in
https://github.com/rails/rails/commit/f766abd4cf3eb75469d3646cfb6d85e668c619f3
caused `scoping` to leak in some tests when run in isolation (looks like
a concurrency issue?). `relation_scoping_test.rb` is a case that failed.
From what I can tell it should not be possible, but changing to the
block form fixes it. There is a deeper issue that I can't seem to find.
/cc @senny
Anchor should not be appended when set to nil/false.

Fixes https://github.com/rails/rails/issues/17714.
Add changelog entry for .reflections API change

`.reflections` public API changed to return a String instead of a Symbol
as keys.

see commit 1f31488499111fdfce79d8dc1cc8fb008f7cdb25 and 6259e4e2dcca9a79f22f96658c33efe81936bc0d

[fixes #16928]
[fixes #17610]
Update reflections public API doc
Merge pull request #17718 from arthurnn/fixes_16928

Fixes 16928, improve reflections public API doc [ci skip]
Fix explanation for ActionDispatch::Callbacks.

ActionDispatch::Callbacks dose not run the prepare callbacks,
so change with comment on ActionDispatch::Callbacks.
Merge pull request #17719 from yui-knk/fix/rails_on_rack_guide

Fix explanation for ActionDispatch::Callbacks. [ci skip]
[ci skip] Dispatching is a verb :bow:

cc #17719
[ci skip] Fix comment of ActionDispatch::Callbacks

cc ddce3dd
Merge pull request #17722 from yui-knk/fix/callbacks_comment

[ci skip] Fix comment of ActionDispatch::Callbacks
Merge pull request #17715 from tgxworld/fix_nil_anchor

Anchor should not be appended when set to nil/false.
Test using `ActionController::TestCase` with engines

Reference #17453

[Godfrey Chan, Washington Luiz]
Correct example in the engines testing guide [ci skip]

Reference #17453