Deprecate redundant AbstractPlatform methods

[morozov]

Q	A
Type	deprecation
BC Break	no

This PR primarily deprecates the AbstractPlatform methods that produce the SQL fragments that don't vary across supported platforms (e.g. getCountExpression() → COUNT()). The additional reasons to deprecate them are:

1. They are not used by the DBAL itself. E.g. IS NULL is used as SQL everywhere instead of using getIsNullExpression().
2. Most of these methods aren't covered by any tests.
3. These methods don't provide any consistent API. E.g. there are methods for SIN(), COS() and ACOS() but there's no ASIN(), TAN() and so on. Implementing a full set of trigonometric functions in a DBAL would be weird.

It also deprecates a bunch of supports*() methods that describe the features consistently implemented across all the supported platforms (i.e. always return the same value).

Additionally, it documents some more deprecations of the code that is already removed in 4.0.x:

1. The methods that support the GUID type. The type was deprecated earlier in DBAL 2.8 (see Deprecated usage of DB-generated UUIDs #3212) reverted in Revert GUID type deprecation #4726.
2. The methods that provide the default and the maximum column lengths on a given platform (see Get rid of hard-coded default values of maximum field lengths #3586).