Bump fmt and spdlog dependencies

[fknorr]

Required to build with recent DPC++.

Recent spdlog inspects CMAKE_CXX_STANDARD, so we need to set this variable. I'm removing CELERITY_CXX_STANDARD in favor of this global setting, and automatically re-apply it in find_package(Celerity) unless the user has requested a higher standard themselves.

[github-actions]

Check-perf-impact results: (000e2892abb21ddd1ae413a5c86d7d95)

❓ No new benchmark data submitted. ❓
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[coveralls]

Pull Request Test Coverage Report for Build 12889454238

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.104%

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Totals
Change from base Build 12789820097:	0.0%
Covered Lines:	7084
Relevant Lines:	7189

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💛 - Coveralls

[fknorr]

Passes the nightly build now!

[GagaLP]

LGTM! Is now working out of the Box with DPCPP, nice :)

[PeterTh]

LGTM!
Thanks for taking care of this, nice to be back to green on nightlies.

[github-actions]

Check-perf-impact results: (deac89ab3c981780086a8f2d08cf6170)

❓ No new benchmark data submitted. ❓
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[github-actions]

Check-perf-impact results: (cad5fb03670f51e00b16688b9ca8eaf9)

✔️ No significant performance change in the microbenchmark set. You are good to go!

Relative execution time per category: (mean of relative medians)

• command-graph : 1.00x
• graph-nodes : 1.02x
• grid : 0.99x
• instruction-graph : 1.00x
• scheduler : 1.00x
• system : 1.04x
• task-graph : 1.00x