Cleanup availability warnings and fix deployment targets.

[lolgear]

It seems that SecKeyAlgorithm ( on which whole library depends ) is introduced in fresh apple OSes.

typedef CFStringRef SecKeyAlgorithm CF_STRING_ENUM
__OSX_AVAILABLE(10.12) __IOS_AVAILABLE(10.0) __TVOS_AVAILABLE(10.0) __WATCHOS_AVAILABLE(3.0);

So, we need to raise deployment targets and cleanup availability warnings.

• Cleanup availability warnings.
• Raise deployment targets.