The minimum macOS supported version is 10.11 as of Go 1.14, see #23011.
Thus, bump macosx-version-min to 10.11
While at it, drop __MAC_OS_X_VERSION_MAX_ALLOWED as suggested by
Filippo:
In general, I can see why we'd want to tell the libraries which
minimum version we target so they drop compatibility with older
versions. No idea why we'd specify a max version, unless it's to make
sure we don't use APIs added after that version, but then it would
have to be 1011 not 1015.
Let's try dropping that define and see if anything blows up? ¯\_(ツ)_/¯