This is because etcd v3.5 will panic when it encounters
ClusterVersionSet entry with version >3.5.0. For downgrades to v3.5 to
work we need to make sure this entry is snapshotted.
The io/ioutil package has been deprecated as of Go 1.16, see
https://golang.org/doc/go1.16#ioutil. This commit replaces the existing
io/ioutil functions with their new definitions in io and os packages.
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>