Change crlShards schema to not have auto_increment primary key

The crlShards table schema is currently
https://github.com/letsencrypt/boulder/blob/a2141cb695de88970bd843aaee91bd8a3c605be5/sa/db/boulder_sa/20230519000000_CrlShards.sql#L4-L13

I now believe that this schema was a mistake. This table does not grow arbitrarily and does not need to be partitioned, so the existence of an auto_increment primary key is a waste.

Instead, this should be a simple key-value table, where the keys are of the form fmt.Sprintf(%d:%d", issuerID, idx). The overall schema should be something like:

CREATE TABLE `crlShards` (
  `id` tinytext NOT NULL,
  `thisUpdate` datetime,
  `nextUpdate` datetime,
  `leasedUntil` datetime NOT NULL,
  PRIMARY KEY (`id`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

When this work is done, the lease functions which select-and-uodate this table should be changed to use SELECT...FOR UPDATE as suggested by the InnoDB doc.

Start with sa/db/boulder_sa/20230519000000_CrlShards.sql and inspect the lease functions that select and update crlShards. Verify the key format and transaction behavior against the MySQL InnoDB documentation, then update the schema and lease queries so the table uses the specified string key and SELECT...FOR UPDATE.