Revert "Update soong's go.mod to 1.15.6"

This reverts commit 23f69ae958da451d31fb0b42f49e05632bccdb57.

Reason for revert: invalid go version '1.15.6': must match format 1.23

Change-Id: Ibe4335f4d9b3ff4b08b4615b91935cca4643ee89
diff --git a/go.mod b/go.mod
index 117fa65..7297dea 100644
--- a/go.mod
+++ b/go.mod
@@ -8,4 +8,4 @@
 
 replace github.com/google/blueprint v0.0.0 => ../blueprint
 
-go 1.15.6
+go 1.15