To formalize they are now XML-compliant (with some asterisks.)
Also to help syntax highlighters work on them to make their content more
readable.
Also:
- Delete empty comment decorations.
- GHA/checksrc: simplify XML check.
- runtests: fail to load test data with XML prolog missing.
Follow-up to bfe6eb1c06#19927
Follow-up to 87ba80a6dfCloses#19946
Only actually needed servers should be listed and none is then implied
if no servers are listed.
Outputs a warning if "none" is still set as a server.
Closes#18466
`runtests.pl` defaults to `-` if a command is not set, since
c43ad0f972.
Also:
- drop a stray `</file>`.
- replace a `nothing` with a guaranteed invalid hostname.
Ref: 4334033b43
- replace unused URLs with `-`.
Closes#18319
