Migrate to using SANs for webhook certificates for Go 1.15

[markmandel]

Context: #1897

Go 1.15 no longer supports Common Name based certs for HTTPS without a specific flag. Common Name is a 20 year old deprecation, and we probably shouldn't be promoting.

There is a backward compatible environment flag, but it will be going away eventually. This flag, for the life of me I could not make it work -- which blocked a migration to latest 1.15.

I tried to write steps to create certs with SANs, but my OpenSSL knowledge is not good enough.

So, as a plan to migrate to Go 1.15:

1. Update webhook with cert creation instructions to use SANs over Common Name, and ensure it works with current Agones version.
2. Provide release note announcement on deprecation of Common Name certificates for webhooks.
3. Migrate build system to Go 1.15 and no longer support SAN based certificates.

[markmandel]

We could really move to 1.16 at this point!
https://golang.org/doc/go1.16

[markmandel]

Worth noting - if you upgrade to 1.16, it's possible the certs we use for validation and mutation webhooks when installing thorugh index.yaml may not container SAN's and may therefore break the install path for install.yaml

helm should be fine, since it generates SAN powered certs.

Example:
https://github.com/googleforgames/agones/blob/main/install/helm/agones/templates/extensions.yaml#L71