Sendable fixup for 1.4.3

The 1.4.3 release carefully introduced Sendable across the library; sadly we missed that 5.6.x Swift series treat a "missing marker protocol conformance for Sendable" as an error while it is intended to be a warning as which it is correctly reported in Swift 5.7.

This release fixes this by not requiring that values stored in Logger.MetadataValue.stringConvertible must be Sendable, however practically speaking they should be thread-safe in any case, as it is not guaranteed in any way when/where this string convertible value will be invoked from.

This release contains no other changes from 1.4.3.

What's Changed

• [sendable] Sendable conformance checks cause errors on 5.6 but warnings on 5.7 by @ktoso in https://github.com/apple/swift-log/pull/229

Full Changelog: https://github.com/apple/swift-log/compare/1.4.3...1.4.4