Followup to #1306

## Problem

if we run `linkerd mc link` using the latest CLI and attempted to apply the resulting manifests into a cluster with linkerd-multicluster on version edge-24.9.2 and earlier we would get the following error:

```
Error from server (BadRequest): error when creating "STDIN": Link in version "v1alpha1" cannot be handled as a Link: strict decoding error: unknown field "spec.probeSpec.failureThreshold", unknown field "spec.probeSpec.timeout"
```

## Solution

This changes the `linkerd mc link` command so that it doesn't generate default entries for the new (optional) fields `failureThreshold` and `timeout`, so that the output remains compatible with previous versions of the Link CRD.

`linkerd mc link` will however still generate those fields for clusters that have an up-to-date multicluster extension. Those fields values continue to be retrieved from the gateway service's `mirror.linkerd.io/probe-failure-threshold` and `mirror.linkerd.io/probe-timeout` annotations.

Note that once the linkerd-multicluster extension gets upgraded in the cluster to match the CLI version, the existing Link CRs will automatically receive the default values for these fields.

…rresponding annotations from the gateway workload