Prepare for 0.9.0 release

CHANGELOG.md

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+<a id='changelog-0.9.0'></a>
+
+## 0.9.0 (2024-07-25)
+
+### Backwards-incompatible changes
+
+- `SquarebotSlackMessageValue` no longer includes a `bot_id` field. Instead, the `user` field is set to the bot's ID and a new `is_bot` field indicates if the `user` field is a bot or user ID.
+
+### New features
+
+- Add support for Slack messages with `subtype` of `bot_message`. This is implemented in the Slack message model `SlackMessageEventContent` as a new `subtype` field that now takes a `SlackMessageSubtype` enum. The Kafka representation of this message, `SquarebotSlackMessageValue` has been updated to now set the `user` attribute to either the user's ID or the bot's ID as appropriate. A new flag, `is_bot` indicates if the `user` field reflects a user or bot (app integration) identifier.
+
+- `SlackMessageEventContent`, a part of the `SlackkMessageEvent` model, now has support for `attachments`. Slack attachments are deprecated for Block Kit, but are still widely used by app integrations. The `attachments` field is a list of `SlackMessageAttachment` objects. These attachments can have text content or individual fields that have titles and values.
+
+- The `SquarebotSlackMessageValue` Kafka message model's `text` field now includes the combined content of the message and its attachments. This makes it easy for consumers to process message content without needing to check for attachments and Block Kit fields. For consumers that _are_ sensitive to the exact structure of the message, the `slack_event` field is still available, which is the original JSON representation of the Slack message, which can be parsed into a `SlackMessageEvent` object.
+
+### Other changes
+
+- Adopt `ruff-shared.toml` from https://github.com/lsst/templates
+
+- The noxfile now uses `uv` as the backend for pip and pip-compile tasks.
+
+- Added a new weekly GitHub Actions CI workflow to validate that the app will run with future versions of dependencies.
+
 <a id='changelog-0.8.0'></a>
+
 ## 0.8.0 (2024-02-28)
 
 ### Backwards-incompatible changes