crypto/x509: bump minimum macOS version to 10.11

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)_/¯

Change-Id: I6b76623a9404724ccda40311ff95b3475ae8a60c
Reviewed-on: https://go-review.googlesource.com/c/go/+/214059


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>