Stop getting service monitor CRD from GitHub (#4855)

## Motivation

Validator deployments can fail with HTTP 429 errors when helmfile tries
to download the Prometheus ServiceMonitor CRD from
raw.githubusercontent.com during the prepare hook. When this happens, it
blocks the entire deployment.

## Proposal

Bundle the ServiceMonitor CRD (v0.86.1) locally in the repository at
`kubernetes/linera-validator/crds/servicemonitor.yaml` instead of
downloading it from GitHub at deploy time.

ServiceMonitor CRDs change infrequently and are designed to be backward
compatible, so using a static version is appropriate. The CRD file
includes documentation with version info and instructions for updating
when needed (typically when upgrading prometheus-operator or quarterly
checks).

Changes:
- Added ServiceMonitor CRD file with documentation header explaining
version, source, and update instructions
- Updated helmfile.yaml to apply the CRD from the local file instead of
the GitHub URL
- Maintains existing check to skip installation if CRD already exists on
the cluster

## Test Plan

- Manually tested: Deploy to fresh cluster - CRD installs successfully
- Verified: helmfile hook completes without GitHub rate limit errors
- Confirmed: ServiceMonitor resources are created correctly after
deployment

## Release Plan

- These changes should be backported to the latest `testnet` branch,
then
    - be released in a validator hotfix.

Author: ndr-ds