Set Resource Group process_mode in UI

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Context

A resource group allows deployment jobs in different pipelines to be run one after the other. The process_mode of a resource group dictates the order of when the deployment jobs are run: unordered (default), oldest_first (starting from the oldest pipeline), and newest first (starting from the newest pipeline).

The process_mode can only be updated through the API. This can grow inconvenient if a customer is dealing with multiple projects.

See: #435437 (comment 1711919114)

Proposed Change

Allow the process_mode of a resource group to be set in the gitlab-ci.yml, ie:

build:
  stage: build
  resource_group:
    name: resource_group_1
    process_mode: oldest_first