YugabyteDB doesn't support ALTER TABLE .. DROP CONSTRAINT IF EXISTS

[lukaseder]

Our API claims that YugabyteDB supports ALTER TABLE .. DROP CONSTRAINT IF EXISTS but this isn't the case:
https://docs.yugabyte.com/preview/api/ysql/the-sql-language/statements/ddl_alter_table/

[lukaseder]

This seems to work:

do $$
begin
  alter table "t" drop constraint "x";
exception
  when sqlstate '42704' then null;
end $$

[lukaseder]

We cannot rely on the documentation. Other syntax that works isn't documented either, see:

• [Docs] Missing documentation for ALTER TABLE .. ADD COLUMN IF NOT EXISTS and DROP COLUMN IF EXISTS yugabyte/yugabyte-db#20354

[lukaseder]

Fixed in versions:

• 3.20.0
• 3.19.1 (YugabyteDB doesn't support ALTER TABLE .. DROP CONSTRAINT IF EXISTS - Merge [#15958] #15959)
• 3.18.8 (YugabyteDB doesn't support ALTER TABLE .. DROP CONSTRAINT IF EXISTS - Merge [#15958] #15960)
• 3.17.17 (YugabyteDB doesn't support ALTER TABLE .. DROP CONSTRAINT IF EXISTS - Merge [#15958] #15961)
• 3.16.23 (YugabyteDB doesn't support ALTER TABLE .. DROP CONSTRAINT IF EXISTS - Merge [#15958] #15962)