coverage.py: Guard use of --html-nested behind version check.

[amcn]

--html-nested was added to gcovr in version 6.0 so passing it to versions before this will result in gcovr complaining that it doesn't recognise the argument.

Added a version check to ensure that we pass a recognised argument for versions before 6.0

Fixes #13781

[amcn]

@eli-schwartz Apologies for the explicit ping, but it seems that others have now started to encounter the underlying bug in the wild: dftd4/dftd4#264 (comment)

Could this fix be reviewed and (if it's acceptable) merged for the next release?

The logs for the Windows CI failure have expired at this point, but IIRC it was the two hour timeout that caused it to fail.

[dcbaker]

I tired to kick the azure pipelines, but they don't want to be kicked, a force push might be required to get them going again

[amcn]

Oh joy, the Windows CI passes now in just over an hour, but the cross-compile armhf job failed, with the following error:

  ERROR: Clock skew detected. File /__w/meson/meson/b 473e503e48/../test cases/common/273 both libraries/meson.build has a time stamp 0.0063s in the future.

Which seems to be a sporadic. I can re-run this again if it's desired.

[eli-schwartz]

@eli-schwartz Apologies for the explicit ping, but it seems that others have now started to encounter the underlying bug in the wild

No problem, you can always feel free to ping me if you think something slipped under the radar.

In this case, something did :) when you originally submitted this PR, I was offline for the holiday season and there were quite a few things that built up and I didn't notice, so pinging me now means I notice it.