fix: datetime.utcfromtimestamp is deprecated and scheduled for removal

[WilliamBergamin]

datetime.utcfromtimestamp was deprecated in python 3.12. It is used in this project in

• slack_sdk/oauth/installation_store/models/installation.py
• slack_sdk/oauth/installation_store/models/bot.py
• slack_sdk/oauth/state_store/sqlalchemy/init.py

We should replace

datetime.utcfromtimestamp(self.installed_at)

In favor of

datetime.fromtimestamp(self.installed_at, tz=timezone.utc)

Unit test should be added to ensure backwards compatibility

Requirements

For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. 🙇

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