ENG-8624 fix operating_system_version/operating_system mismatch (tink…

…erbell#275)

Signed-off-by: Kelly Deng <kelly@packet.com>

## Description

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This PR fixes the mismatch in field names [`operating_system_version`](https://github.com/tinkerbell/tink/blob/master/protos/packet/packet.proto#L86) vs [`operating_system`](https://github.com/tinkerbell/hegel/blob/a3138d417536903dcdedd674d634e13ebc19fc79/http_handlers.go#L33).

## Why is this needed
This bug mainly affects the users who needs both the `packet.pb.go` and the EC2 endpoint. Because the EC2 endpoint in Hegel uses the field name `operating_system` to be backwards compatible with Kant, users using the `Metadata` object defined in `packet.pb.go` will face a problem with parsing the operating system field since it was defined as `operating_system_version` there. 
The reason for the mismatch is because in the "old" cacher mode, `operating_system_version`, but Kant uses `operating_system` (which we have to be backwards compatible with) so we decided move away from `operating_system_version` and go with `operating_system` for "tink mode".
A user could have worked around this by specifying two fields `operating_system` and `operating_system_version` with the same value, inside the same piece of hardware, but this introduces some redundancy and just isn't ideal.

Related PR: tinkerbell/smee#104

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Fixes: #

## How Has This Been Tested?
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To be manually tested

## How are existing users impacted? What migration steps/scripts do we need?

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Existing users who import the `packet.pb.go` will have to `go get` latest master of the tink repo.

## Checklist:

I have:

- [ ] updated the documentation and/or roadmap (if required)
- [ ] added unit or e2e tests
- [ ] provided instructions on how to upgrade

protos/packet/packet.proto

@@ -83,7 +83,7 @@ message Metadata {
     bool allow_pxe = 4;
     bool rescue = 5;
 
-    OperatingSystem operating_system_version = 6;
+    OperatingSystem operating_system = 6;
     bool always_pxe = 7;
     string ipxe_script_url = 8;
     repeated IP ips = 9;