Deprecate keychain support (#164)

Keychain support can be removed to simplify the library, considering
that there are numerous ways to use an environment variable without
storing the key as plain-text. For example, using password manager CLIs
or exporting from keychain to variable each time `TOKEN=$(security ...)
my-script.main.kts`.

library/src/main/kotlin/com/gabrielfeo/gradle/enterprise/api/internal/Keychain.kt

@@ -26,9 +26,15 @@ internal class RealKeychain(
         if (status != 0) {
             return KeychainResult.Error("exit $status")
         }
+        println(KEYCHAIN_DEPRECATION_WARNING)
         val token = process.inputStream.bufferedReader().use {
             it.readText().trim()
         }
         return KeychainResult.Success(token)
     }
 }
+
+private const val KEYCHAIN_DEPRECATION_WARNING =
+    "WARNING: passing token via macOS keychain is deprecated. Please pass it as the " +
+        "GRADLE_ENTERPRISE_API_TOKEN environment variable instead. Keychain support will be " +
+        "removed in the next release. See release notes for details and alternatives."