docs: add Go-version requirement column for Swap (1.20) and Clear (1.23)

[millerjp]

Summary

README API table at line 100+ lists Swap and Clear without their Go version requirement. A consumer pinned to Go 1.21 who uses Clear hits a build error with no warning in the table they consulted.

Scope

Add a Notes-column annotation to the Swap and Clear rows ("Requires Go 1.20" / "Requires Go 1.23"). Also mention in doc.go's Relationship section.

Acceptance criteria

1. README API table cites Go version minima on Swap and Clear.
2. doc.go notes that CompareAndSwap/CompareAndDelete also require Go 1.20.

Source: docs-writer.