gRPC Swift 1.13.2

SemVer Patch

• Update swift-nio-ssl dependency (#1541, #1543, patch credit to @oliverepper)
• Avoid an unnecessary copy in the client read state (#1533)
• Avoid ack-ing pings twice (#1534)
• Add a coalescing writer (#1537, #1539, #1546)
• Remove unused code (#1549)
• Bump version number to 1.13.2 (#1550)

Other Changes

• Allow Swift Package Index to generate documentation (#1535, #1536)
• Update perf test precondition (#1545)
• Use 5.8-nightly in CI (#1547)