Make AS::SafeBuffer#prepend act like String#prepend

Make `#prepend` method modify instance in-place and return self
instead of just returning modified value. That is exactly what
`#prepend!` method was doing previously, so it's deprecated from
now on.
PostgreSQL, Support for materialized views. [Dave Lee & Yves Senn]

Expand the query used in #table_exists? to include materialized views in the
kinds of relations it searches.
We can conditional define the tests depending on the adapter or
connection.
Merge pull request #14565 from rajcybage/conditional_test_cases

We can conditional define the tests depending on the adapter or connection.
DRY AS::SafeBuffer a bit using existing helper
Fixed small documentation typo

Using `Rails.application.config.cookies_serializer :hybrid` throws an exception, fixed to `=`.
Merge pull request #14568 from alex88/patch-1

Fixed upgrade guide typo
Merge pull request #14529 from rwz/master

ActiveSupport::SafeBuffer#prepend inconsistency
Use asert_deprecated
Move require to actual file

Change to require all active_support/deprecation since that's the actual
entry point for the deprecation methods.
Fix cookie serializer docs

The option actually needs to be set on action_dispatch. [ci skip]
Fix default_url_options example in i18n guide [ci skip]
Merge pull request #14564 from killthekitten/patch-2

Fix default_url_options example in i18n guide