bump clang-format to clang-15

[kroening]

clang 11 was released in 2007, and the last variant of Ubuntu with the clang-format-11 package is 22.04.

This bumps the clang-format used for the formatting check to clang 15, available with 24.04.

• 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/
• n/a 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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.95%. Comparing base (36b2335) to head (9030c33).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8561      +/-   ##
===========================================
+ Coverage    78.79%   78.95%   +0.15%     
===========================================
  Files         1730     1730              
  Lines       199160   198759     -401     
  Branches     18320    18314       -6     
===========================================
  Hits        156938   156938              
+ Misses       42222    41821     -401     

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