Remove the ability to rename columns with alter_column operations (#…

…603)

Remove the ability to rename columns with `alter_column` operations.

#602 added the `rename_column` operation as the preferred way to rename
columns:

```json
{
  "name": "13_rename_column",
  "operations": [
    {
      "rename_column": {
        "table": "employees",
        "from": "role",
        "to": "job_title"
      }
    }
  ]
}
```

As of this PR, `rename_column` operations are the only way to rename
columns, with the ability to do so using `alter_column` operations
removed.

This is a breaking change to the `pgroll` migrations format.

This is the second (and final) part of #601 

Closes #601

internal/jsonschema/testdata/alter-column-2.txtar

@@ -1,5 +1,5 @@
 This is a valid 'alter_column' migration.
-It sets `name` and therefore does not specify `up` or `down`.
+It sets both `type` and `nullable`.
 
 -- alter_column.json --
 {
@@ -9,7 +9,10 @@ It sets `name` and therefore does not specify `up` or `down`.
       "alter_column": {
         "table": "reviews",
         "column": "review",
-        "name": "new_name"
+        "type": "integer",
+        "nullable": true,
+        "up": "foo",
+        "down": "foo"
       }
     }
   ]

internal/jsonschema/testdata/alter-column-3.txtar

@@ -1,5 +1,5 @@
 This is an invalid 'alter_column' migration.
-It sets only `name` but also sets `up` and `down`.
+It specifies a table and column but no changes.
 
 -- alter_column.json --
 {
@@ -8,10 +8,7 @@ It sets only `name` but also sets `up` and `down`.
     {
       "alter_column": {
         "table": "reviews",
-        "column": "review",
-        "name": "new_name",
-        "up": "foo",
-        "down": "foo"
+        "column": "review"
       }
     }
   ]

pkg/migrations/errors.go

@@ -174,18 +174,6 @@ func (e ConstraintAlreadyExistsError) Error() string {
 	return fmt.Sprintf("constraint %q on table %q already exists", e.Constraint, e.Table)
 }
 
-type NoUpSQLAllowedError struct{}
-
-func (e NoUpSQLAllowedError) Error() string {
-	return "up SQL is not allowed for this operation"
-}
-
-type NoDownSQLAllowedError struct{}
-
-func (e NoDownSQLAllowedError) Error() string {
-	return "down SQL is not allowed for this operation"
-}
-
 type BackfillNotPossibleError struct {
 	Table string
 }