fix: enable decK's convert subcommand to work with Konnect again

[GGabriele]

After the deprecation of the old deck konnect command and the old Konnect format, the deck convert command with the Konnect format is not useful anymore in its current shape because the old Konnect format is not compatible anymore with the new workflow handled directly with the main deck command.

Despite the new Konnect format is mostly compatible with the 'regular' Kong format, it has a few differences:

• it needs the _format_version entry set to "3.0"
• it doesn't support the _workspace entry

This means that doing a deck dump and then a deck sync from one Gateway with workspaces to Konnect may not natively work because of these incompatibilities.

This commit makes sure to support the new Konnect format in the deck convert command. The new convertion does the following:

• set the _format_version entry to "3.0"
• handle the removal of the _workspace entry and the addition of the _konnect.runtime_group_name in the following manner:
  1. if present, remove the _workspace entry
  2. if --konnect-runtime-group-name foo flag is used, then set _konnect.runtime_group_name: foo
  3. if no --konnect-runtime-group-name flag is used, then set _konnect.runtime_group_name to the value _workspace was precedently set to
  4. if no _workspace was present and no --konnect-runtime-group-name flag is used, then leave the whole _konnect entry unset