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/.gitignore b/.gitignore
index e1b9f6d..d76279a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -120,6 +120,8 @@
 third_party/kotlin/kotlin-compiler-1.5.0
 third_party/kotlin/kotlin-compiler-1.6.0.tar.gz
 third_party/kotlin/kotlin-compiler-1.6.0
+third_party/kotlin/kotlin-compiler-1.7.0.tar.gz
+third_party/kotlin/kotlin-compiler-1.7.0
 third_party/kotlin/kotlin-compiler-dev.tar.gz
 third_party/kotlin/kotlin-compiler-dev
 third_party/kotlinx-coroutines-1.3.6.tar.gz