chore: Bump to Go version 1.21.5

Go 1.21.5 fixes a few vulnerabilities. Most notably for boundary is a
fix in net/http to limit chunked data overhead.

See: https://groups.google.com/g/golang-announce/c/iLGK3x6yuNo
(cherry picked from commit 575cd9d63b)

.go-version

@@ -1 +1 @@
-1.21.4
+1.21.5

api/go.mod

@@ -2,7 +2,7 @@ module github.com/hashicorp/boundary/api
 
 go 1.21
 
-toolchain go1.21.4
+toolchain go1.21.5
 
 require (
 	github.com/hashicorp/boundary/sdk v0.0.40

sdk/go.mod

@@ -2,7 +2,7 @@ module github.com/hashicorp/boundary/sdk
 
 go 1.21
 
-toolchain go1.21.4
+toolchain go1.21.5
 
 require (
 	github.com/hashicorp/eventlogger v0.2.6-0.20231025104552-802587e608f0