Bump cmake version to 3.8

[thomasspriggs]

This PR bumps the cmake version to 3.8. Because this is the minimum cmake version which supports a value of 17 for CXX_STANDARD. This enables moving the C++ standard version we use for compiling cbmc to C++17 based on setting this setting rather than setting compiler flags.

See also related PR - #7172

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• 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/
• 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).
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[thomasspriggs]

Reference to the cmake documentation for CXX_STANDARD - https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html
CMake 3.8 was released in November 2018, so it should be relatively widely available by now - https://github.com/Kitware/CMake/releases/tag/v3.8.0

[codecov]

Codecov Report

Base: 77.82% // Head: 77.89% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (7f86707) compared to base (2a08d59).
Patch coverage: 91.45% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7184      +/-   ##
===========================================
+ Coverage    77.82%   77.89%   +0.07%     
===========================================
  Files         1578     1616      +38     
  Lines       182242   186701    +4459     
===========================================
+ Hits        141824   145433    +3609     
- Misses       40418    41268     +850     

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.cpp	95.11% <90.47%> (-1.18%)	⬇️
src/assembler/remove_asm.cpp	78.66% <93.10%> (+27.28%)	⬆️
src/goto-instrument/contracts/contracts.h	100.00% <100.00%> (ø)
src/goto-instrument/nondet_static.cpp	96.92% <100.00%> (ø)
src/goto-programs/string_instrumentation.cpp	25.11% <100.00%> (ø)
src/solvers/smt2/smt2_conv.h	84.61% <0.00%> (-15.39%)	⬇️
src/util/invariant.cpp	72.54% <0.00%> (-7.85%)	⬇️
src/goto-programs/goto_program.h	90.19% <0.00%> (-0.51%)	⬇️
src/cprover/instrument_contracts.cpp	90.51% <0.00%> (ø)
src/cprover/state_encoding_targets.cpp	5.46% <0.00%> (ø)
... and 48 more

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