Fix double minor version additions in update images GHA

[wpross]

• Only create one update per major.minor version showing the final patch version jump
  The resulting PR shows

   - name: cloud-controller-manager
     sourceRepository: github.com/kubernetes-sigs/cloud-provider-azure
     repository: mcr.microsoft.com/oss/kubernetes/azure-cloud-controller-manager
     tag: v1.34.0
     labels:
     - name: gardener.cloud/cve-categorisation
       value:
         network_exposure: protected
         authentication_enforced: false
         user_interaction: gardener-operator
         confidentiality_requirement: high
         integrity_requirement: high
         availability_requirement: low
       signing: false
     targetVersion: 1.34.x
  - name: cloud-controller-manager
     sourceRepository: github.com/kubernetes-sigs/cloud-provider-azure
     repository: mcr.microsoft.com/oss/kubernetes/azure-cloud-controller-manager
     tag: v1.34.1
     labels:
     - name: gardener.cloud/cve-categorisation
       value:
         network_exposure: protected
         authentication_enforced: false
         user_interaction: gardener-operator
         confidentiality_requirement: high
         integrity_requirement: high
         availability_requirement: low
       signing: false
     targetVersion: '>= 1.34'

  but it should only add the last of the two image entries since targetVersion: '>= 1.34' includes tag: v1.34.0.
• Only show final patch version in summary that's used as release notes
  The release notes show:

    - cloud-controller-manager: v1.33.3 -> v1.34.0 (minor)
    - cloud-controller-manager: v1.34.0 -> v1.34.1 (minor)
  

  Since v1.34.0 should not be in images.yaml, it's better not to mention it as a minor update, so the code is changed in order to not print the 1st line in such a case. For consistency reasons (and since it makes more sense), the same style is applied to patch updates, so intermediate updates that do not end up in the images.yaml are not mentioned anymore.
• Change headings formatting for better readability

Release note:

Fix double minor version additions in update-extension-provider-images action.