[patch] Fix Sonar issues in ApiUrlTestTask

[DanProgs <Abenteurer.2901@…>]

The deprecated URL constructor in ApiUrlTestTask has been updated, and the exception handling has been adjusted (changing MalformedURLException to IllegalArgumentException | MalformedURLException). The recurring string "Connection to API failed" has been defined as a constant, and the responseCode has been stored in a variable. Furthermore, curly braces have been added to control structures, and fields and variables have been declared as final.

see: https://josm.openstreetmap.de/sonar/project/issues?files=src%2Forg%2Fopenstreetmap%2Fjosm%2Fgui%2Fpreferences%2Fserver%2FApiUrlTestTask.java&issueStatuses=CONFIRMED%2COPEN&open=AYtXfRgJQ0Q6PFw6n23f&id=josm
see: https://josm.openstreetmap.de/sonar/project/issues?files=src%2Forg%2Fopenstreetmap%2Fjosm%2Fgui%2Fpreferences%2Fserver%2FApiUrlTestTask.java&issueStatuses=CONFIRMED%2COPEN&open=3a548e29-540e-47f2-bffe-c156ab7ef302&id=josm
see: https://josm.openstreetmap.de/sonar/project/issues?files=src%2Forg%2Fopenstreetmap%2Fjosm%2Fgui%2Fpreferences%2Fserver%2FApiUrlTestTask.java&issueStatuses=CONFIRMED%2COPEN&open=7142450a-1760-4b2b-89cd-10abce9fdb63&id=josm