Update external repo links that changed default branch to main

Signed-off-by: Tony Fang <nhfang@amazon.com>

code-of-conduct.md

@@ -1,3 +1,3 @@
 ## containerd Community Code of Conduct
 
-containerd follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).
+containerd follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/main/code-of-conduct.md).

docs/cri/config.md

@@ -301,7 +301,7 @@ version = 2
       #
       # For the naming convention of annotation keys, please reference:
       # * Kubernetes: https://kubernetes.io/docs/concepts/overview/working-with-objects/annotations/#syntax-and-character-set
-      # * OCI: https://github.com/opencontainers/image-spec/blob/master/annotations.md
+      # * OCI: https://github.com/opencontainers/image-spec/blob/main/annotations.md
       pod_annotations = []
 
       # container_annotations is a list of container annotations passed through to the OCI config of the containers.

docs/rootless.md

@@ -15,7 +15,7 @@ $ containerd-rootless-setuptool.sh install
 $ nerdctl run -d --restart=always --name nginx -p 8080:80 nginx:alpine
 ```
 
-See https://github.com/containerd/nerdctl/blob/master/docs/rootless.md for the further information.
+See [nerdctl/docs/rootless.md](https://github.com/containerd/nerdctl/blob/main/docs/rootless.md) for further information.
 
 ## "Hard way"
 
@@ -33,7 +33,7 @@ $ rootlesskit --net=slirp4netns --copy-up=/etc --copy-up=/run \
 ```
 
 * `--net=slirp4netns --copy-up=/etc` is only required when you want to unshare network namespaces.
-  See [RootlessKit documentation](https://github.com/rootless-containers/rootlesskit/blob/v0.14.1/docs/network.md) for the further information about the network drivers.
+  See [RootlessKit documentation](https://github.com/rootless-containers/rootlesskit/blob/v0.14.1/docs/network.md) for further information about the network drivers.
 * `--copy-up=/DIR` mounts a writable tmpfs on `/DIR` with symbolic links to the files under the `/DIR` on the parent namespace
   so that the user can add/remove files under `/DIR` in the mount namespace.
   `--copy-up=/etc` and `--copy-up=/run` are needed on typical setup.

pkg/netns/netns_linux.go

@@ -46,7 +46,7 @@ import (
 )
 
 // Some of the following functions are migrated from
-// https://github.com/containernetworking/plugins/blob/master/pkg/testutils/netns_linux.go
+// https://github.com/containernetworking/plugins/blob/main/pkg/testutils/netns_linux.go
 
 // newNS creates a new persistent (bind-mounted) network namespace and returns the
 // path to the network namespace.