Update Chef version requirement from 16+ to 18+

[Copilot]

After PR #778, the cookbook now requires Chef 18+ due to Ruby 3.1 features, but the documentation still stated Chef 16+. This creates a mismatch between the actual requirements and what users see in the documentation.

Background

PR #778 ("feat: Support PostgreSQL 17") introduced a .rubocop.yml file with TargetRubyVersion: 3.1, which effectively requires Ruby 3.1+ features. The Chef-to-Ruby version mapping is:

• Chef 16: Ruby 2.7
• Chef 17: Ruby 3.0
• Chef 18: Ruby 3.1

Since the cookbook now targets Ruby 3.1, it requires Chef 18 or later to function properly.

Changes

• Updated metadata.rb: Changed chef_version from '>= 16.0' to '>= 18.0'
• Updated README.md: Changed Chef requirement from "Chef 16" to "Chef 18"

This ensures consistency between the cookbook metadata (shown on Chef Supermarket) and the documentation, preventing users from attempting to use the cookbook with incompatible Chef versions.

Fixes #790.

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Slowest examples

Top 10 slowest examples (0.01 seconds, 47.65% of total time)

Example	Description	Time in seconds
spec/libraries/helpers_spec.rb:19	PostgreSQL::Cookbook::Helpers#data_dir(version) with rhel family and Postgres 11 from repo returns the correct path	0.01253
spec/libraries/helpers_spec.rb:43	PostgreSQL::Cookbook::Helpers#data_dir(version) with debian family and Postgres 11 from os returns the correct path	0.00042
spec/libraries/helpers_spec.rb:119	PostgreSQL::Cookbook::Helpers#default_platform_service_name(version) with debian family and Postgres 14 from os returns the correct service name	0.00041
spec/libraries/helpers_spec.rb:111	PostgreSQL::Cookbook::Helpers#default_platform_service_name(version) with debian family and Postgres 15 from repo returns the correct service name	0.00039
spec/libraries/helpers_spec.rb:95	PostgreSQL::Cookbook::Helpers#default_platform_service_name(version) with rhel family and Postgres 11 from repo returns the correct service name	0.00038
spec/libraries/helpers_spec.rb:27	PostgreSQL::Cookbook::Helpers#data_dir(version) with rhel family and Postgres 11 from os returns the correct path	0.00038
spec/libraries/helpers_spec.rb:73	PostgreSQL::Cookbook::Helpers#conf_dir(version) with debian family and Postgres 15 from repo returns the correct path	0.00038
spec/libraries/helpers_spec.rb:81	PostgreSQL::Cookbook::Helpers#conf_dir(version) with debian family and Postgres 12 from os returns the correct path	0.00038
spec/libraries/helpers_spec.rb:65	PostgreSQL::Cookbook::Helpers#conf_dir(version) with rhel family and Postgres 11 from os returns the correct path	0.00038
spec/libraries/helpers_spec.rb:73	PostgreSQL::Cookbook::Helpers#conf_dir(version) with debian family and Postgres 14 from repo returns the correct path	0.00038