Trim cruft and harden the Docker gateway idempotency fix

[m-czernek]

Porting saltstack/salt@6f146d6 which was not picked up by the migration.

• systemd_service: drop the else-raise added to enable()/disable()'s sysvinit paths. It's unreachable (_get_service_exec() only ever returns a path ending in update-rc.d or chkconfig, or raises first) and untested; its commit rationale actually described the _runlevel() IndexError, which is fixed separately.
• dockermod.compare_networks: tighten the auto-assigned-Gateway exception so it only ignores a one-sided Gateway when it equals the subnet's Docker- assigned default (network address + 1). Previously any one-sided Gateway was stripped unconditionally, which would mask a real change if a user explicitly set a non-default gateway and then removed it. Add unit tests covering the auto-assigned case, the non-default-gateway-removed case, and a changed-explicit-gateway case, mirroring the existing test_compare_networks_ipam_config_empty_iprange regression test. Add a changelog entry describing the fix accurately: the auto-gateway/no-gateway mismatch is structural (any Docker version that auto-assigns gateways), not specific to a newer Docker engine.

Fixes #22

[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.07%. Comparing base (cebd4c0) to head (9680a3c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/saltext/dockermod/modules/dockermod.py	53.33%	11 Missing and 3 partials ⚠️
tests/unit/modules/dockermod/test_module.py	26.31%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
- Coverage   42.10%   42.07%   -0.03%     
==========================================
  Files          26       26              
  Lines        6545     6590      +45     
  Branches     1138     1145       +7     
==========================================
+ Hits         2756     2773      +17     
- Misses       3542     3567      +25     
- Partials      247      250       +3     

Flag	Coverage Δ
Linux	42.06% <42.85%> (-0.03%)	⬇️
project	37.76% <53.33%> (+0.05%)	⬆️
py310	42.01% <42.85%> (-0.03%)	⬇️
py311	42.01% <42.85%> (-0.03%)	⬇️
py314	42.01% <42.85%> (-0.03%)	⬇️
salt_3006_27	42.01% <42.85%> (-0.03%)	⬇️
salt_3008_2	42.01% <42.85%> (-0.03%)	⬇️
tests	50.17% <26.31%> (-0.21%)	⬇️

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