upgrade of DROP CONSTRAINT

Related dev. issue(s): https://github.com/tarantool/tarantool/issues/9112

Product: Tarantool
Since: 3.0
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_sql/sql_statements_and_clauses/#alter-table
SME: @ ImeevMA

Details

Now, instead of just ALTER TABLE table DROP CONSTRAINT constraint; we
have 8 operator variants:

1. Statement to drop PRIMARY KEY, UNIQUE, tuple FOREIGN NEY or tuple
   CHECK constraints:

ALTER TABLE tab_name DROP CONSTRAINT constr_name;

This statement cannot drop a constraint if constr_name matches
more than one constraint.

2. Statement to drop field FOREIGN NEY or field CHECK constraints:

ALTER TABLE tab_name DROP CONSTRAINT field_name.constr_name;

This statement cannot drop a constraint if constr_name matches
more than one constraint for the field_name field.

3. Statement to drop PRIMARY KEY constraint:

ALTER TABLE tab_name DROP CONSTRAINT constr_name PRIMARY KEY;

4. Statement to drop UNIQUE constraint:

ALTER TABLE tab_name DROP CONSTRAINT constr_name UNIQUE;

5. Statement to drop tuple FOREIGN KEY constraint:

ALTER TABLE tab_name DROP CONSTRAINT constr_name FOREIGN KEY;

6. Statement to drop tuple CHECK constraint:

ALTER TABLE tab_name DROP CONSTRAINT constr_name CHECK;

7. Statement to drop field FOREIGN KEY constraint:

ALTER TABLE tab_name DROP CONSTRAINT field_name.constr_name FOREIGN KEY;

8. Statement to drop field CHECK constraint:

ALTER TABLE tab_name DROP CONSTRAINT field_name.constr_name CHECK;

Requested by @ ImeevMA in https://github.com/tarantool/tarantool/commit/263777dcdd49dd7c48d4b54d9f5996f7802d9837.

Start with the ALTER TABLE section in the linked Tarantool SQL reference, then compare its DROP CONSTRAINT syntax with the eight variants listed in this issue. Update that section to document the supported forms and their constraint-selection rules; done means all eight statements and ambiguity restrictions are accurately represented.