Fix Ruuvi Gateway data being ignored when system is not using UTC time

[akx]

Proposed change

The stale data comparison in the Ruuvi Gateway integration did the Wrong Thing around timezones, so if your system timezone is UTC+anything, all data was and ignored.

I'm not sure why this worked with HA 38f183a (#84853) and has now broken.

This should probably be released in a bugfix version of 2023.2.

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)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR is related to issue: Ruuvi Gateway integration not working #87143

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 dev checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

[akx]

@bdraco I can switch this switched this to use UNIX time comparisons – that should be both faster and more bulletproof?