doc: update the minimum supported macOS version to 10.11
doc: update the minimum supported macOS version to 10.11

Update minimum macOS supported version from 10.10 to 10.11.

Updates #23011

Change-Id: Ie10c40e882c9d309ff56041d9768afc288d0204f
Reviewed-on: https://go-review.googlesource.com/c/go/+/213878
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

akutz/go,golang/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,golang/go,nawawi/go,CAFxX/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,golang/go,akutz/go,nawawi/go,nawawi/go,golang/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,akutz/go,akutz/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go