Rename `overpass.txt` to `overpass.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.)
Also, it is the most widely used extension on GitHub, see Rename to `*.overpassql` (view)

I propose to rename all Overpass QL 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: github-linguist/linguist#5890

[FloEdelmann]

@samsaurio Would a pull request be accepted?