Change migration guide stub generator to pages

Now it should generate the appropriate front matter for pages in
migration guide section rather than sections.

generate-release/src/migration_guide.rs

@@ -21,9 +21,6 @@ pub fn generate_migration_guide(
         r#"+++
 title = "{title}"
 weight = {weight}
-sort_by = "weight"
-template = "docs-section.html"
-page_template = "docs-section.html"
 insert_anchor_links = "right"
 [extra]
 long_title = "Migration Guide: {title}"
@@ -104,7 +101,6 @@ As a result, the Minimum Supported Rust Version (MSRV) is "the latest stable rel
 
     println!("\nFound {} breaking PRs merged", count);
 
-
     std::fs::write(path, output)?;
 
     Ok(())