Consolidate API requests and update ICS locations and descriptions for multi-state holidays

[Skillkiller]

The API requests have been updated to query all federal states simultaneously, consolidating the data into a single raw JSON. As a result, the architecture has been simplified to use just one override file and one output result JSON. This change makes it possible to display information about holidays in other states when generating ICS files, which resolves #28.

To achieve this, each holiday now features a Location field containing the abbreviations of all applicable states. In individual state ICS files, the target state's information is prioritized first, followed by the remaining states sorted alphabetically. Furthermore, the event description has been enhanced to explicitly show where the holiday applies: it either states that the holiday is nationwide or lists the specific states in a list.

Examples

For a Baden-Württemberg (BW) file, the location field prioritizes BW first:

LOCATION:BW\, BY\, BE\, BB\, HB\, HH\, HE\, MV\, NI\, NW\, RP\, SL\, SN\, ST\, SH\, TH

For a Mecklenburg-Vorpommern (MV) file, MV is placed at the beginning:

LOCATION:MV\, BW\, BY\, BE\, BB\, HB\, HH\, HE\, NI\, NW\, RP\, SL\, SN\, ST\, SH\, TH

When a holiday applies nationwide, the description is set as follows:

DESCRIPTION:Gilt in allen Bundesländern.

If the holiday is regional, the description lists the specific states:

DESCRIPTION:Gilt in:\n- Baden-Württemberg\n- Bayern\n- Sachsen-Anhalt