remove depricated Validatior#setup
docs, refactor docs about column modifiers. [ci skip] [Matthew Draper & Yves Senn]

This is a follow up to #15602 which rendered the guides in a weird state:

  >  You can also specify some options just after the field type between curly
     braces. You can use the following modifiers:

  >  `null`         Allows or disallows `NULL` values in the column.

  >  NOTE: `null` and `default` cannot be specified via command line.

The modifiers are now moved into a separate section. The generator
simply referes to that section.

Related to #15583.

/cc @JuanitoFatas
test, fix typo, `create_index` does not exist.
Fix cache_digest rake tasks

Bring cache_digests:* rake tasks up-to-date with the API changes introduced in
637bb726cac60aaa1f7e482836458aa73e17fbb7
Inline PG array type casting helper
Merge pull request #15607 from sgrif/sg-array-type-cast

Inline PG array type casting helper
Merge pull request #15605 from mekishizufu/fix_cache_digests_rake_tasks

Fix cache_digest rake tasks
Keep the types of virtual columns after yaml serialization

On MySQL and PostgreSQL, the adapter does not type cast virtual columns
for us.
Merge pull request #15603 from kuldeepaggarwal/remove-deprecation-method

remove depricated Validatior#setup
Merge pull request #15609 from sgrif/sg-yaml-virtual-columns

Keep the types of virtual columns after yaml serialization
Silence warnings in tests.

```
activerecord/test/cases/adapters/postgresql/range_test.rb:159: warning: (...) interpreted as grouped expression
activerecord/test/cases/finder_test.rb:38: warning: shadowing outer local variable - e
activerecord/test/cases/finder_test.rb:43: warning: shadowing outer local variable - e
```
Merge pull request #15610 from sgrif/silence-warnings

Fix warnings in tests
[ci skip] Correct output and use valid options of number helpers
Remove deprecated method ActiveRecord::Base.quoted_locking_column
Merge pull request #15611 from akshay-vishnoi/doc_changes

[ci skip] Correct output and use valid options of number helpers
[ci skip] Add Docs for strip_insignificant_zeros option in number helpers
Merge pull request #15612 from akshay-vishnoi/delete-deprecated-method

Remove deprecated method ActiveRecord::Base.quoted_locking_column
[ci skip] Improve enable_starttls_auto documentation.
fix error with long keys in ActiveSupport::Cache::FileStore
Merge pull request #15614 from akshay-vishnoi/doc_changes

[ci skip] Add Docs for strip_insignificant_zeros option in number_to_human