[analyzer] Rename the element model migration guide.

The new name clarifies that the guide is specifically about element
model migration, not migration in general.

Changing the name now allows us to make links to the guide without
having to worry about updating those links if we change the name
later.

Change-Id: I6860867246b41671c20ecc725fdb2a5f7b77b949
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/419240
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer/doc/migration_guide.md renamed to pkg/analyzer/doc/element_model_migration_guide.md

@@ -60,8 +60,10 @@ class SnippetTester {
   }
 
   Future<void> verifyFile(File file) async {
-    if (file.path.endsWith('/pkg/analyzer/doc/migration_guide.md') ||
-        file.path.endsWith(r'\pkg\analyzer\doc\migration_guide.md')) {
+    if (file.path
+            .endsWith('/pkg/analyzer/doc/element_model_migration_guide.md') ||
+        file.path
+            .endsWith(r'\pkg\analyzer\doc\element_model_migration_guide.md')) {
       return;
     }
     String content = file.readAsStringSync();