Prepare 0.2.0 release (API breaking changes) (#43)

### Motivation

It's time to release an API-breaking version and delete all the
deprecated code.

### Modifications

Deleted all the deprecated code.

This is a breaking change, so the integration test will fail until we
land this and then the corresponding generator changes.

### Result

Removed a bunch of code!

### Test Plan

Let's let CI check everything except for the integration test.

README.md

@@ -15,7 +15,7 @@ Add the package dependency in your `Package.swift`:
 ```swift
 .package(
     url: "https://github.com/apple/swift-openapi-runtime",
-    .upToNextMinor(from: "0.1.0")
+    .upToNextMinor(from: "0.2.0")
 ),
 ```
 

Sources/OpenAPIRuntime/Conversion/CurrencyExtensions.swift

@@ -15,13 +15,7 @@ import Foundation
 
 extension HeaderField: CustomStringConvertible {
     public var description: String {
-        let value: String
-        if HeaderField.internalRedactedHeaderFields.contains(name.lowercased()) {
-            value = "<redacted>"
-        } else {
-            value = self.value
-        }
-        return "\(name): \(value)"
+        "\(name): \(value)"
     }
 }