override charset on duo state column to ascii for mysql

migrations/mysql/2024-06-05-131359_add_2fa_duo_store/up.sql

@@ -1,5 +1,8 @@
 CREATE TABLE twofactor_duo_ctx (
-    state      VARCHAR(1024) NOT NULL,
+    -- For mysql, the character set on state is overridden to ascii because the utf8mb4 database charset recommended in
+    -- the Vaultwarden docs causes 1 character to consume 4 bytes, exceeding innodb's 3072 max key size if we want to
+    -- accommodate the largest supported state size. This isn't a problem for nonce since it's not a key for the table.
+    state      VARCHAR(1024) CHARACTER SET ascii COLLATE ascii_general_ci NOT NULL,
     user_email VARCHAR(255)  NOT NULL,
     nonce      VARCHAR(1024) NOT NULL,
     exp        BIGINT        NOT NULL,