Update release-data JSON file format (#274)

This makes the format open for extension, such as adding release cycle level data (such as EOL dates).

Version data is still accessible by the version's name. While this repeats the version name, it's also much more convenient for users of those data.

A few other things have also been updated in the process:

- verbosity of the diff has been increased in update.py to make workflow summaries more readable,
- dates without timezone are now set to UTC by default (this was already supposed, so no impact expected here).

src/common/dates.py

@@ -44,8 +44,14 @@ def parse_datetime(text: str, formats: list[str] = frozenset([
     text = text.strip().replace(", ", " ").replace(". ", " ").replace("(", "").replace(")", "")
     for fmt in formats:
         try:
-            date = datetime.strptime(text, fmt)  # NOQA: DTZ007, timezone is handled below
-            return date.astimezone(timezone.utc) if to_utc else date
+            dt = datetime.strptime(text, fmt)  # NOQA: DTZ007, timezone is handled below
+
+            if to_utc:
+                dt = dt.astimezone(timezone.utc)
+            elif dt.tzinfo is None:
+                dt = dt.replace(tzinfo=timezone.utc)
+
+            return dt
         except ValueError:
             pass