osxkeychain: match min macos version for xx

[crazy-max]

partially solves #280

xx sets a minimal macos version if not set through MACOSX_VERSION_MIN env var: https://github.com/tonistiigi/xx/blob/3d00d096c8bf894ec29bae5caa5aea81d9c187a5/base/xx-info#L203-L209

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0b91805) 55.23% compared to head (5944f8a) 55.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #283 +/- ## ======================================= Coverage 55.23% 55.23% ======================================= Files 9 9 Lines 668 668 ======================================= Hits 369 369 Misses 256 256 Partials 43 43 ``` | [Impacted Files](https://app.codecov.io/gh/docker/docker-credential-helpers/pull/283?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=docker) | Coverage Δ | | |---|---|---| | [osxkeychain/osxkeychain\_darwin.go](https://app.codecov.io/gh/docker/docker-credential-helpers/pull/283?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=docker#diff-b3N4a2V5Y2hhaW4vb3N4a2V5Y2hhaW5fZGFyd2luLmdv) | `69.10% <ø> (ø)` | |

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[thaJeztah]

Haven't tried yet, but wondering if we need to update these two lines;

https://github.com/docker/docker-credential-helpers/blob/f8e94d91c0f3bafeddf19f03454c23112b0312ea/osxkeychain/osxkeychain_darwin.go#L4-L5

[crazy-max]

Haven't tried yet, but wondering if we need to update these two lines;

https://github.com/docker/docker-credential-helpers/blob/f8e94d91c0f3bafeddf19f03454c23112b0312ea/osxkeychain/osxkeychain_darwin.go#L4-L5

I think we can keep 10.11 as minimum version looking at the current implementation of the keychain in our codebase.