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.
Changed 'so' to 'see'
Allow postgresql enum_test to be run in random order.

Creating and dropping similar tables within the same connection causes postgresql to look up old values in the cache of tables which have already been dropped.
Merge pull request #14560 from abwinkler999/typo-in-RELEASING_RAILS

Changed 'so' to 'see'
Merge pull request #14550 from tgxworld/fix_failing_enum_test_random_order

Allow postgresql enum_test to be run in random order.
cleanup, `reset_pg_session` in range_test.rb

Also do not use transactional fixtures. We drop the type and the
table after every run, so there is nothing for the transaction to clean up.
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]