Modernize RainMachine config flow

[bachya]

Proposed change

This PR modernizes the RainMachine config flow:

• Removes OrderedDict usage (no longer needed)
• Uses built-in unique ID and abort helpers (instead of custom logic)
• Registers the domain upon config flow definition

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml
simplisafe:
  accounts:
    - username: !secret simplisafe_username
      password: !secret simplisafe_password

Additional information

• This PR fixes or closes issue: N/A
• This PR is related to issue: N/A
• Link to documentation pull request: N/A

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

[codecov]

Codecov Report

Merging #32131 into dev will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev   #32131      +/-   ##
==========================================
- Coverage   94.74%   94.74%   -0.01%     
==========================================
  Files         767      767              
  Lines       55498    55493       -5     
==========================================
- Hits        52581    52575       -6     
- Misses       2917     2918       +1

Impacted Files	Coverage Δ
...omeassistant/components/rainmachine/config_flow.py	97.05% <100%> (-0.38%)	⬇️
homeassistant/components/uk_transport/sensor.py	93.47% <0%> (-0.73%)	⬇️

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