feat: Support floating tag in product image selection

[Techassi]

This PR adds support for auto updating product images. This is done by using a floating tag for product images instead of fully-qualified ones.

Significant changes are:

• Add a new autoUpdate field to the product image selection CRD interface:

  autoUpdate:
    default: false
    description: |-
      Automatically update the product image. Defaults to `false`.
  
      This mechanism utilizes a floating image tag which always refers to the latest patch version
      in the current release line. The current release line is either automatically derived, or
      can be overridden via `stackableVersion`.
  
      It should be noted that when this field is set to `true`, the operator uses `Always` as the
      pull policy for product images. If set to `false`, `IfNotPresent` is used. Explicitly
      setting `pullPolicy` takes precedence.
  
      ### Examples
  
      - The `stackableVersion` field is not set, the operator falls back to its own version, eg.
        26.7.0. If this field is set to `true`, the `26.7` floating tag will be used for product
        images, else, `26.7.0` will be used.
      - The `stackableVersion` field is set to `26.3.0`. If this field is set to `true`, the
        `26.3` floating tag will be used for product images, else, `26.3.0` will be used.
    type: boolean

• Change the default pullPolicy from Always to IfNotPresent. This better aligns with the
  auto update feature, because if enabled, the pull policy is automatically set to Always. The
  current default is (as far as I can see) a leftover from the past. Even the doc comment states
  that Always is the default, as we support floating tags (not true).

• The stackableVersion field is now a proper semantic version, instead of a plain string. This is
  needed, to correctly extract the floating tag if the auto update is enabled.

This PR also serves as a decision. Please vote with 👍 or 👎 or leave comments.