Fix datetime.datetime.utcnow() deprecation warning.

Python 3.12 will print the following warning when using
`datetime.datetime.utcnow()`:

  DeprecationWarning: datetime.datetime.utcnow() is deprecated and
  scheduled for removal in a future version. Use timezone-aware objects
  to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).

Follow the suggestion, except we don't use `datetime.UTC` to avoid
breaking compatibility with Python 3.6.

full_offline_backup_for_todoist/backup_downloader.py

@@ -21,7 +21,7 @@ def download(self, vfs: VirtualFs) -> None:
 
         # Sanitize the file name for platforms such as Windows,
         # which don't accept some characters in file names, such as a colon (:)
-        backup_version = datetime.datetime.utcnow().replace(microsecond=0).isoformat(' ')
+        backup_version = datetime.datetime.now(datetime.timezone.utc).strftime("%Y-%m-%d %H:%M:%S")
         vfs.set_path_hint(sanitize_file_name("TodoistBackup_" + backup_version))
 
         # Download the file