Bump cmake minimum version to 3.13

[TheMagnetar]

Having the minimum cmake version lower than 3.13 will leave policies CMP0079 and CMP0077 unset. This will have conflicts with other projects that use cppzmq with add_subdirectory and that have a dependency on Catch2 as well to fail to compile.

Bumping the cmake_minimum_version to 3.13 prevents that.

[gummif]

Can cmake_policy be used instead to not break things?

[coveralls]

Pull Request Test Coverage Report for Build 19046429772

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.379%

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Totals
Change from base Build 17909983277:	0.0%
Covered Lines:	905
Relevant Lines:	1024

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💛 - Coveralls

[TheMagnetar]

I will also work, note that however other policies may still be left unset. What solution do you find more suited for this project?

In my opinion bumping the minimum required version is cleaner than defining a set of policies manually. However I do not have a good perspective where cppzmq is used nor the requirements of other projects. Therefore I will implement your suggestion.