SWIFT-855 Add minimum macOS version and document OS support policy

[kmahar]

This adds a minimum macOS version to our package manifest, and clarifies supported / tested platforms in our README.
This also lets us remove the availability checks for 10.12+ we had to use for the date formatter.

[codecov-commenter]

Codecov Report

Merging #487 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #487   +/-   ##
=======================================
  Coverage   76.89%   76.89%           
=======================================
  Files         125      125           
  Lines       13012    13012           
=======================================
  Hits        10005    10005           
  Misses       3007     3007           

Impacted Files	Coverage Δ
Sources/MongoSwift/ChangeStream.swift	30.64% <ø> (ø)
Sources/MongoSwift/ClientSession.swift	39.30% <ø> (ø)
Sources/MongoSwift/MongoClient.swift	87.09% <ø> (ø)
...ces/MongoSwift/MongoCollection+ChangeStreams.swift	43.75% <ø> (ø)
Sources/MongoSwift/MongoCollection+Indexes.swift	89.33% <ø> (ø)
Sources/MongoSwift/MongoCollection+Read.swift	100.00% <ø> (ø)
Sources/MongoSwift/MongoCursor.swift	100.00% <ø> (ø)
Sources/MongoSwift/MongoDatabase.swift	80.35% <ø> (ø)
.../MongoSwift/Operations/StartSessionOperation.swift	95.45% <ø> (ø)
Sources/MongoSwiftSync/ChangeStream.swift	0.00% <ø> (ø)
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8435316...eb47cce. Read the comment docs.