Add kotlin 1.7 to our kotlin test matrix

This will also move kotlin 1.4.20 into the old bots testing.

Contents of README.google:

Name: Kotlin
URL: https://github.com/JetBrains/kotlin/
Version: 1.7.0
Revision: NA
License: Apache License Version 2.0

Change-Id: Ifa042fc9fbcaf132231ea80d8c2fcbbed7cd7253
diff --git a/build.gradle b/build.gradle
index 7c3743f..5b9c8df 100644
--- a/build.gradle
+++ b/build.gradle
@@ -346,6 +346,7 @@
                 "kotlin/kotlin-compiler-1.4.20",
                 "kotlin/kotlin-compiler-1.5.0",
                 "kotlin/kotlin-compiler-1.6.0",
+                "kotlin/kotlin-compiler-1.7.0",
                 "kotlinx-coroutines-1.3.6",
                 "openjdk/openjdk-rt-1.8",
                 "openjdk/desugar_jdk_libs",