Improve/simplify file headers

[sunib]

All the headers in source files still mention copyright 2025, adjust them to 2026 please.

[scott-david-walker]

Is there any reason why we want to have the headers on a file level basis? There is already a license file in the root of the project that covers this?

[sunib]

When I introduced this I read that it was wise to attribute every file, but it looks indeed like 80s stuff. I've been re-reading a bit and I think that it is fine to just replace the whole header with the single machine readable spdx line

// SPDX-License-Identifier: Apache-2.0

That's a nice suggestion let's just do that!