bump OS X version

[kroening]

This increases the required version of OS X from 10.9 to 10.15, which is the
oldest version of OS X that Apple still supplies with security fixes.

• 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.
• 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

Merging #6379 (eebdba8) into develop (54a50e9) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6379      +/-   ##
===========================================
- Coverage    75.91%   75.91%   -0.01%     
===========================================
  Files         1517     1517              
  Lines       164024   164022       -2     
===========================================
- Hits        124515   124513       -2     
  Misses       39509    39509              

Impacted Files	Coverage Δ
src/util/cmdline.h	66.66% <ø> (ø)
src/util/cmdline.cpp	95.32% <100.00%> (-0.06%)	⬇️

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[chrisr-diffblue]

Seems sensible to bump this.