Mark unique_id() virtual method as deprecated

[oxan]

What does this implement/fix?

These old-style methods are only present for (text) sensors and not for all other types of entities. They are overridden and implemented by only a handful of sensors; the vast majority use the automatic unique ID generation. We'll likely never be able to remove them due to backwards compatibility, but let's stop the bleeding and mark them as deprecated so that we don't get any more components that implement them.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040

Example entry for config.yaml:

# Example config.yaml

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

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

• Documentation added/updated in esphome-docs.

[probot-esphome]

Hey there @esphome/core, mind taking a look at this pull request as it has been labeled with an integration (sensor) you are listed as a code owner for? Thanks!
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