[20.0][MIG] base_user_role: Migration to 20.0

[alan196]

Mirror of the upstream migration OCA#488 plus fixes:

• Role groups are light in Odoo 20 (res.groups._get_light_group_xmlids()): a role group is a container like base.default_user_group, so it must not turn its users into regular users by itself; the light/regular tier now comes from the groups implied by the roles. The override returns the role group ids (SetDefinitions.get_id() resolves ids too) instead of creating __custom__ XML IDs with _ensure_xml_id() from a compute path, and drops the arbitrary limit=1000. The _apply_group_regular() calls on role create/unlink from the upstream PR are kept.
• Tests: restored the original assertions of test_create_role_from_user and test_group_groups_into_role (no need to discard base.group_user_regular anymore), added test_role_group_is_light (light-only role → light_user, role implying a regular group → regular_user, group back to regular after the role is deleted) and test_user_form_role_readonly (native role radio read-only when the user has role lines, as requested in the upstream review).
• FontAwesome icons (fa-book, fa-users, fa-gears) replaced with the 20.0 icon-font names (lock, group, settings).
• _inherits = {"res.groups": "group_id"} # noqa: RUF012 instead of the ClassVar annotation objected to in the upstream review (ruff RUF012 is enabled by the repo pre-commit).

The upstream commits are untouched; the fixes are the two last commits.

Upstream CI is red only because of the codecov upload step (SSL handshake failure); both test jobs there report 0 failed, 0 error(s).

Tested on Odoo 20.0 (odoo/odoo 20.0, database with demo data):

• -i base_user_role --test-enable --test-tags /base_user_role: 28 tests, 0 failed, 0 errors, no deprecation warnings.
• -u base --test-tags /base:TestGroupsOdoo,/base:TestUsers2,/base:TestUsers,/base:TestResUsersForm with the module installed (light/regular group invariants of core): 52 tests, 0 failed, 0 errors.
• Repo pre-commit: all hooks passed.

Assisted-by: Claude Fable 5.1

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Additional details and impacted files

@@           Coverage Diff           @@
##             20.0       #6   +/-   ##
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  Coverage        ?   89.39%           
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  Files           ?       12           
  Lines           ?      509           
  Branches        ?       39           
=======================================
  Hits            ?      455           
  Misses          ?       44           
  Partials        ?       10           

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