Rename `*.osm` files to `*.overpassql`

[FloEdelmann]

*.overpassql is now the recommended file extension for Overpass QL: OSM Wiki: Overpass QL § File extension (Disclaimer: I added that section after some discussion.)

I propose to rename all *.osm files in this repo to *.overpassql for consistency.

Context: GitHub's syntax highlighting engine requires that 200 repositories use the same file extension before they consider supporting that syntax and file extension: https://github.com/github/linguist/issues/5890