Fix false-positive primary key column length deprecation

[morozov]

This PR fixes false-positive primary key column length deprecation introduced in #6787. Originally, it would trigger if the array of column lengths passed to a primary key index is not empty. This is too strict since during introspection lengths are populated with null values by default:

dbal/src/Schema/AbstractSchemaManager.php

Line 857 in 113744f

$result[$keyName]['options']['lengths'][] = $row['length'] ?? null;

Instead, we want to trigger the deprecation only if a non-null length would be associated with an indexed column.

Additionally, I want to introduce two more deprecation-related changes:

1. Deprecate extending the Index class similar to UniqueConstraint and ForeignKeyConstraint.
2. Annotate all methods accepting a list of index columns as accepting a non-empty list.

I had to add a couple of @phpstan-ignores because even though Index and UniqueConstraint accept a non-empty list of column names, they initialize their corresponding properties with an empty array and then populate them by calling a protected method. So this property has to be initialized with an empty array and thus cannot be guaranteed to be non-empty.