Refactor upgrade role to not include "specification" at top level

[to-bar]

Describe the bug
In upgrade role there are include_vars tasks used without the name parameter what loads variables at top level with higher precedence than task vars.

So having in upgrade role:

- name: Node Exporter | Include specification vars from node_exporter role
  include_vars:
    file: roles/node_exporter/vars/main.yml

causes that specification of node_exporter is global with high precedence.

Then the following task (postgresql upgrade is run after node_exporter) has unexpected content (from node_exporter) in specification var:

- name: repmgr for PG {{ pg_version }} | Replace repmgr config file
  template:
    src: repmgr.conf.j2
    dest: "{{ upgrade_defaults.repmgr.config_dir[ansible_os_family] }}/repmgr.conf"
    owner: postgres
    group: postgres
    mode: u=rw,g=,o=
  vars:
    specification:
      extensions:
        replication:
          replication_user_name: "{{ postgresql_manifest.specification.extensions.replication.replication_user_name }}"
          repmgr_database:       "{{ postgresql_manifest.specification.extensions.replication.repmgr_database }}"

Expected behavior
Task include_vars should be used with name parameter, for example:

- name: Node Exporter | Include specification vars from node_exporter role
  include_vars:
    file: roles/node_exporter/vars/main.yml
    name: node_exporter_vars

then specification would not override task vars but be available under node_exporter_vars.specification.

Additional context
Having globally loaded vars with high precedence may cause difficult to debug errors.

For local vars we can use underscore in names, e.g. _specification.

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DoD checklist

• Changelog updated (if affected version was released)
• COMPONENTS.md updated / doesn't need to be updated
• Automated tests passed (QA pipelines)
  • apply
  • upgrade
• Case covered by automated test (if possible)
• Idempotency tested
• Documentation updated / doesn't need to be updated
• All conversations in PR resolved
• Backport tasks created / doesn't need to be backported

[przemyslavic]

✔️ Tested upgrade with affected components enabled (kafka, node exporter, logging, rabbitmq, repmgr for postgresql).