[4.0.0 future major release] Deprecated postgresql_set module in favor of postgresql_alter_system module

[Andersson007]

SUMMARY

Deprecated postgresql_set module in favor of postgresql_alter_system module:

• Update its docs
• Add a warning when postgresql_set is used
• Announce under deprecated_feature
• Create an issue under milestone 5.0.0 to remove the module in 5.0.0, but not earlier than 2026.