[REF] data: rename legacy migration file

Done in a different commit to let git understand and preserve the file
history.

Author: LucasLefevre

src/index.ts

@@ -139,7 +139,7 @@ import {
   createEmptySheet,
   createEmptyWorkbookData,
 } from "./migrations/data";
-import { legacyMigrationStepRegistry } from "./migrations/migration_steps";
+import { legacyMigrationStepRegistry } from "./migrations/legacy_migration_steps";
 import { migrationStepRegistry } from "./migrations/new_migration_steps";
 import {
   corePluginRegistry,

src/migrations/data.ts

@@ -12,7 +12,7 @@ import {
   WorkbookData,
 } from "../types/index";
 import { XlsxReader } from "../xlsx/xlsx_reader";
-import { legacyMigrationStepRegistry } from "./migration_steps";
+import { legacyMigrationStepRegistry } from "./legacy_migration_steps";
 import { migrationStepRegistry } from "./new_migration_steps";
 
 const INITIAL_SHEET_ID = "Sheet1";