[feat] Add PlacementGroupRef to linodeMachine

[tchinmai7]

What this PR does / why we need it:

1. Adds a placementGroupRef to the linodeMachine controller
2. bump linodego to 1.38
3. fix placementgroup api breaking change

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

1. To test this, add the following to the generated manifest ( the placementGroupRef and LinodePlacementGroup)

---
apiVersion: infrastructure.cluster.x-k8s.io/v1alpha2
kind: LinodeMachineTemplate
metadata:
  name: schinmai-test-control-plane
  namespace: default
spec:
  template:
    spec:
      authorizedKeys: null
      image: linode/ubuntu22.04
      interfaces:
      - purpose: public
      placementGroupRef:
        apiVersion: infrastructure.cluster.x-k8s.io/v1alpha2
        kind: LinodePlacementGroup
        name: schinmai-test
      region: us-ord
      type: g6-standard-2
---
apiVersion: infrastructure.cluster.x-k8s.io/v1alpha2
kind: LinodePlacementGroup
metadata:
  name: schinmai-test
  namespace: default
spec:
  region: us-ord
---

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[codecov]

Codecov Report

Attention: Patch coverage is 62.06897% with 11 lines in your changes missing coverage. Please review.

Project coverage is 66.15%. Comparing base (fbb370a) to head (18e939b).

Files	Patch %	Lines
controller/linodemachine_controller_helpers.go	59.25%	7 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #418      +/-   ##
==========================================
+ Coverage   66.03%   66.15%   +0.11%     
==========================================
  Files          64       64              
  Lines        3336     3365      +29     
==========================================
+ Hits         2203     2226      +23     
- Misses        974      977       +3     
- Partials      159      162       +3     

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