CODING_STANDARD.md: fix references to clang-format

[kroening]

This fixes the description of how to use clang-format in CODING_STANDARD.md.

• The CI uses clang-format-11 (as opposed to -10) since 557d941.

• The promise that this is available on Ubuntu 18.04 cannot be met. The earliest version of Ubuntu that has clang-format-11 out of the box is 20.04.

• There is a reference to COMPILING.md, which does not say anything at all about clang-format.

• 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 ✅

Comparison is base (cb62568) 78.80% compared to head (d082399) 77.31%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7995      +/-   ##
===========================================
- Coverage    78.80%   77.31%   -1.50%     
===========================================
  Files         1701     1701              
  Lines       196462   196465       +3     
===========================================
- Hits        154819   151890    -2929     
- Misses       41643    44575    +2932     

see 111 files with indirect coverage changes

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