Clean and optimize systemmonitor

[bdraco]

Breaking change

The last_boot time is now returned in UTC

Proposed change

• Remove unneeded excinfo to _LOGGER.exception
• Use f-strings
• Switch last_boot to utc
• Cache psutil/os calls used by multiple attributes in the same update cycle

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

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Additional information

• This PR fixes or closes issue: fixes #
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[MartinHjelmare]

Looks good!