Increase column width of vault_key on mysql (#14231)

* resolves The default schema used in the mysql backend is insufficient for KVv2 storage #14114
* increases column width of vault_key from 512 to 3072 in mysql physical backend
* updates changelog

changelog/14231.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+ physical/mysql: Create table with wider `vault_key` column when initializing database tables.
+```

physical/mysql/mysql.go

@@ -120,7 +120,7 @@ func NewMySQLBackend(conf map[string]string, logger log.Logger) (physical.Backen
 	// Create the required table if it doesn't exists.
 	if !tableExist {
 		create_query := "CREATE TABLE IF NOT EXISTS " + dbTable +
-			" (vault_key varbinary(512), vault_value mediumblob, PRIMARY KEY (vault_key))"
+			" (vault_key varbinary(3072), vault_value mediumblob, PRIMARY KEY (vault_key))"
 		if _, err := db.Exec(create_query); err != nil {
 			return nil, fmt.Errorf("failed to create mysql table: %w", err)
 		}