Add more zeros to solution file name

[andrewwinters5000]

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

[github-actions[bot]]

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[codecov[bot]]

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 :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1996 +/- ## ======================================= Coverage 96.16% 96.16% ======================================= Files 460 460 Lines 36984 36984 ======================================= Hits 35564 35564 Misses 1420 1420 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1996/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1996/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `96.16% <86.67%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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[JoshuaLampert]

This needs downstream changes in Trixi2Vtk.jl.

[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