rev to 3.0.0

[devoncarew]

• rev to 3.0.0 to account for breaking API changes introduced in 2.4.1 (see 2.4.1 seems to have introduced breaking changes #307 for context)
• adjust the CI to test against both the latest and oldest versions of package dependencies
• misc updates to the readme and docs

This 3.0.0 version is substantially the same as the content from 2.4.1.

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[devoncarew]

We'd need to coordinate with package:test when rolling this out:

Because test >=1.16.3 <1.16.6 depends on web_socket_channel >=1.0.0 <3.0.0 and test >=1.16.0-nullsafety.8 <1.16.3 depends on web_socket_channel ^1.0.0, test >=1.16.0-nullsafety.8 <1.16.6 requires web_socket_channel >=1.0.0 <3.0.0.
And because test >=1.16.6 depends on web_socket_channel ^2.0.0, test >=1.16.0-nullsafety.8 requires web_socket_channel >=1.0.0 <3.0.0.
So, because web_socket_channel depends on test ^1.16.0 and web_socket_channel is 3.0.0, version solving failed.