Latvia - Add missing public holidays - #1089

The law "Par svētku, atceres un atzīmējamām dienām" defines three public holidays that were missing from the provider:

• Final Day of the Nationwide Latvian Song and Dance Celebration - held every five years, date fixed per event by Cabinet order (2018-07-08, 2023-07-09, 2028-07-09). The same Saturday/Sunday observed rule as for 4 May and 18 November applies.
• 2018-09-24 - pastoral visit of Pope Francis (one-off)
• 2023-05-29 - Latvian ice hockey team bronze medal win (one-off)

Also renames 26 December from "St. Stephen's Day" to "Second day of Christmas" (Otrie Ziemassvētki), matching the law and the naming used by the Bulgaria and Svalbard providers; the Id changes from LV-STSTEPHENSDAY-01 to LV-CHRISTMASDAY-02. Adds the official English translation of the law to the sources.

Sources:

• https://likumi.lv/ta/en/en/id/72608-on-public-holidays-commemoration-days-and-celebration-days
• https://likumi.lv/ta/id/367734-par-xxviii-visparejo-latviesu-dziesmu-un-xviii-deju-svetku-norises-laiku (2028 dates)