updata reporting wrong max version in remove-update command

[ytsssun]

Platform I'm building on:
x86_64

What I expected to happen:
updata to report correct max version available in the tuf repo. (In my case it is supposed to be 1.32.0)

What actually happened:
updata reported wrong value. (In my case it showed 1.18.0)

How to reproduce the problem:

1. Download the tuf manifest using tuftool

tuftool download \
    --metadata-url "https://updates.bottlerocket.aws/2020-07-07/aws-k8s-1.29/x86_64/" \
    --targets-url "https://updates.bottlerocket.aws/targets" \
    --root ./default.root.json \
    --target-name manifest.json \
    ./aws-k8s-1.29/x86_64/tuf_in

2. Call updata -- remove-update

updata -- remove-update ./aws-k8s-1.29/x86_64/tuf_in/manifest.json \
  --arch x86_64 \
  --version 1.30.0 \
  --variant aws-k8s-1.29
20:08:47 [INFO] Update x86_64-aws-k8s-1.29-1.30.0 removed. Current maximum version: 1.18.0     <== Wrong version 

Sample manifest.json

{
  "updates": [
    {
      "variant": "aws-k8s-1.29",
      "arch": "x86_64",
      "version": "1.18.0",
      "max_version": "1.32.0",
      ...
    },
    ....
    {
      "variant": "aws-k8s-1.29",
      "arch": "x86_64",
      "version": "1.30.0",
      "max_version": "1.32.0",
      ...
    },
    {
      "variant": "aws-k8s-1.29",
      "arch": "x86_64",
      "version": "1.31.0",
      "max_version": "1.32.0",
      ...
    },
    {
      "variant": "aws-k8s-1.29",
      "arch": "x86_64",
      "version": "1.32.0",
      "max_version": "1.32.0",
      ...
    }
  ],
  "migrations": {
     ...
  }

The max_version in the updates field is showing correct data. But the code is referencing the version field of the first item of the updates, which is why the max version is not correctly captured. The fix could be sort the manifest and get the latest version from the manifest version, or we can simply use the max_version field.