use C++20

[kroening]

This bumps the build from C++17 to C++20.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

❌ Patch coverage is 76.47059% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.83%. Comparing base (0ebdc4c) to head (41d684e).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/format_expr.cpp	54.54%	5 Missing ⚠️
src/util/std_expr.cpp	33.33%	4 Missing ⚠️
src/util/format_type.cpp	0.00%	2 Missing ⚠️
src/goto-programs/goto_trace.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9156      +/-   ##
===========================================
- Coverage    80.83%   80.83%   -0.01%     
===========================================
  Files         1717     1716       -1     
  Lines       190069   190110      +41     
  Branches        73       74       +1     
===========================================
+ Hits        153647   153680      +33     
- Misses       36422    36430       +8     

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