Applications can set their migrations to ignore exceptions raised when adding a column that already exists or when removing a column that does not exist.
Example Usage:
```
class AddColumnTitle <ActiveRecord::Migration[6.1]
Add `Regexp.escape` to one method in ActiveRecord, so that table names with regular expression characters in them work as expected. Since MS SQL Server uses "[" and "]" to quote table and column names, and those characters are regular expression characters, methods like `pluck` and `select` fail in certain cases when used with the MS SQL Server adapter.
* Store advisory locks on their own named connection.
Previously advisory locks were taken out against a connection when a migration started. This works fine in single database applications but doesn't work well when migrations need to open new connections which results in the lock getting dropped.
In order to fix this we are storing the advisory lock on a new connection with the connection specification name `AdvisoryLockBase`. The caveat is that we need to maintain at least 2 connections to a database while migrations are running in order to do this.
* Deprecate `#remove_connection` in favor of `#remove_connection_pool` when called on the handler.
`#remove_connection` is deprecated in order to support returning a `DatabaseConfig` object instead of a `Hash`. Use `#remove_connection_pool`, `#remove_connection` will be removed in 6.2.
* Ensure `:reading` connections always raise if a write is attempted.
Now Rails will raise an `ActiveRecord::ReadOnlyError` if any connection on the reading handler attempts to make a write. If your reading role needs to write you should name the role something other than `:reading`.
* Deprecate "primary" as the connection_specification_name for ActiveRecord::Base
`"primary"` has been deprecated as the `connection_specification_name` for `ActiveRecord::Base` in favor of using `"ActiveRecord::Base"`. This change affects calls to `ActiveRecord::Base.connection_handler.retrieve_connection` and `ActiveRecord::Base.connection_handler.remove_connection`. If you're calling these methods with `"primary"`, please switch to `"ActiveRecord::Base"`.
Freeze the configurations hash to disallow directly changing it. If applications need to change the hash, for example to create databases for parallelization, they should use the `DatabaseConfig` object directly.
* The `connection_config` method has been deprecated, please use `connection_db_config` instead which will return a `DatabaseConfigurations::DatabaseConfig` instead of a `Hash`.
* The `database` kwarg is deprecated without replacement because it can't be used for sharding and creates an issue if it's used during a request. Applications that need to create new connections should use `connects_to` instead.
* Calling methods like `establish_connection` with a `Hash` which is invalid (eg: no `adapter`) will now raise an error the same way as connections defined in `config/database.yml`.
* Add `DatabaseConfig#configuration_hash` to return database configuration hashes with symbol keys, and use all symbol-key configuration hashes internally. Deprecate `DatabaseConfig#config` which returns a String-keyed `Hash` with the same values.
In some cases application authors want to use the database switching middleware and make explicit calls with `connected_to`. It's possible for an app to turn off writes and not turn them back on by the time we call `connected_to(role: :writing)`.
