fix: remove deprecated Ansible templating

[sunlei]

Summary

• replace deprecated access to Ansible's internal vars dictionary with exact ansible.builtin.varnames queries
• construct release API URLs with inline Jinja expressions instead of embedded templates
• apply the URL fix consistently across all exporter and Prometheus roles

Before

With ansible-core 2.21.3, the collection emits these warnings:

[DEPRECATION WARNING]: The internal "vars" dictionary is deprecated.
This feature will be removed from ansible-core version 2.24.
Origin: roles/_common/tasks/preflight.yml:14:9

Use the `vars` and `varnames` lookups instead.

[WARNING]: Jinja constant strings should not contain embedded templates.
This feature will be disabled by default in ansible-core 2.23.
Origin: roles/node_exporter/tasks/preflight.yml:53:28

Use inline expressions instead.

The first warning occurs in three shared preflight checks. The second pattern
occurs in the latest-version lookup of all 25 public roles.

Testing

• ansible-lint
• yamllint roles
• verified the common preflight succeeds without deprecated variables when ANSIBLE_ALLOW_EMBEDDED_TEMPLATES=false
• verified all three deprecated variable checks still reject variables whose value is false