Update Platforms Support Policy in README

The **Support Policy** section of the README includes a bit that states the initial versions from which the "last 4 major versions" policy will apply.

This bit [was recently removed](auth0/Auth0.swift#808) from Auth0.swift's README because it resulted in confusion regarding the currently supported platform versions.

This PR removes it from SimpleKeychain's README as well.

README.md

@@ -147,14 +147,6 @@ The minimum supported Swift minor version is the one released with the oldest-su
 
 ### Platforms
 
-Only the last 4 major platform versions are supported, starting from:
-
-- iOS **12**
-- macOS **10.15**
-- macCatalyst **13**
-- tvOS **12**
-- watchOS **6.2**
-
 Once a platform version becomes unsupported, dropping it from SimpleKeychain **will not be considered a breaking change**, and will be done in a **minor** release. For example, iOS 13 will cease to be supported when iOS 17 gets released, and SimpleKeychain will be able to drop it in a minor release.
 
 In the case of macOS, the yearly named releases are considered a major platform version for the purposes of this Policy, regardless of the actual version numbers.