[firefox][unity] Add support for cumulative updates and use it in a few scripts (#295)

Generic support for cumulative updates has been added to speed up execution time of some scripts that were very long (in comparison with the vast majority of products), usually because they were involving a lot of HTTP requests.

This feature was developed particularily for the firefox.py and unity.py scripts, which was often very long to execute (a minute or moreaccording to GHA summaries). Those scripts has been updated to make use of this new feature.

src/aws-lambda.py

@@ -13,7 +13,6 @@ them though. Note that this would also be unnecessary if it was possible to disa
 release dates updates in the latest.py script."""
 
 with releasedata.ProductData("aws-lambda") as product_data:
-    old_product_data = releasedata.ProductData.from_file(product_data.name)
     product_frontmatter = endoflife.ProductFrontmatter(product_data.name)
     response = http.fetch_url("https://docs.aws.amazon.com/lambda/latest/dg/lambda-runtimes.html")
     soup = BeautifulSoup(response.text, features="html5lib")
@@ -30,7 +29,7 @@ with releasedata.ProductData("aws-lambda") as product_data:
 
             date = product_frontmatter.get_release_date(identifier)  # use the product releaseDate if available
             if date is None:
-                date = old_product_data.get_version(identifier).date()  # else use the previously found date
+                date = product_data.get_previous_version(identifier).date()  # else use the previously found date
             if date is None:
                 date = dates.today()  # else use today's date