Updated install doc of the minimum required macOS

[glennschler]

Testing GMT versions 6.1.1, 6.1.0, and 6.0.0 on macOS version 10.13.6
show only GMT 6.0.0 is able to execute even after successful installation.
The two 6.1.x versions display this error:

/Applications/GMT-6.1.0.app/Contents/MacOS/GMT-6.1.0: line 21: 89507 Abort trap: 6           "${BUNDLE_RESOURCES}/bin/gmt" "$@"
Note 1: If you want to use GMT outside of this terminal or in scripts, then follow these steps:
        a) export GMTHOME=/Applications/GMT-6.1.0.app/Contents/Resources
        b) add $GMTHOME/bin to your path
        c) export PROJ_LIB=$GMTHOME/share/proj
        d) export MAGICK_CONFIGURE_PATH=$GMTHOME/lib/GraphicsMagick/config
Note 2: GMT may use Ghostscript, GraphicsMagick, FFmpeg, and GDAL executables; see
        /Applications/GMT-6.1.0.app/Contents/Resources/share/Licenses for details.

bash-3.2$ gmt
dyld: lazy symbol binding failed: Symbol not found: ____chkstk_darwin
  Referenced from: /Applications/GMT-6.1.0.app/Contents/Resources/bin/../lib/libcurl.4.dylib
(which was built for Mac OS X 10.15)
  Expected in: /usr/lib/libSystem.B.dylib

dyld: Symbol not found: ____chkstk_darwin
  Referenced from: /Applications/GMT-6.1.0.app/Contents/Resources/bin/../lib/libcurl.4.dylib
(which was built for Mac OS X 10.15)
  Expected in: /usr/lib/libSystem.B.dylib

Abort trap: 6

These test results indicate that 6.1.0 and 6.1.1 require a minimum
macOS version of 10.15. GMT version 6.0.0 executes without error
on macOS 10.13.6.

Description of proposed changes

The install instructions for macOS application bundles chart have
been updated to show minimum macOS requirements for the
three GMT releases which were tested.

Resolve #5538

Reminders

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[PaulWessel]

Thanks for the feedback and PR!

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