Set minimum OS X deployment version to 10.14

[darbyjohnston]

Fixes #1718

Compiling the latest OTIO with C++17 support gives this error on macOS 10.15.7:

'any_cast<const opentimelineio::v1_0::AnyVector &>' is unavailable: introduced in macOS 10.14
AnyVector const& av = std::any_cast<AnyVector const&>(source);
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/any:581:12: note:
'any_cast<const opentimelineio::v1_0::AnyVector &>' has been explicitly marked unavailable here

Bumping CMAKE_OSX_DEPLOYMENT_TARGET to 10.14 fixes the issue.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.95%. Comparing base (82068e3) to head (9c434c6). Report is 2 commits behind head on main.

Additional details and impacted files

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