Add more zeros to solution file name

[andrewwinters5000]

This allows the number in the solution file name to max out at 999999999.

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.16%. Comparing base (29eb55a) to head (9ca5803).

Files	Patch %	Lines
src/callbacks_step/save_solution_dg.jl	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1996   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files         460      460           
  Lines       36984    36984           
=======================================
  Hits        35564    35564           
  Misses       1420     1420           

Flag	Coverage Δ
unittests	96.16% <86.67%> (ø)

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[JoshuaLampert]

This needs downstream changes in Trixi2Vtk.jl.

[ranocha]

Thanks a lot! I have updated the other output callbacks and the NEWS.md.

[ranocha]

@sloede Could you please review this PR as well? If you're fine, we can release this in v0.8 and make the appropriate changes to Trixi2Vtk.jl as noticed by @JoshuaLampert

[sloede]

LGTM (I assume you have grep'd for all occurrences of %06d and replaced them by %09d where appropriate; I didn't double-check this)!