Update golang Docker tag to v1.21.6

This MR contains the following updates:

Package	Type	Update	Change
golang	final	minor	1.13.5 -> 1.21.6

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common/docker_ldap/templates/Dockerfile

-FROM golang:1.13.5
+FROM golang:1.21.6
 
 RUN go get github.com/ThoreKr/syncrepl_exporter