Truncate slack block text so it is not rejected by slack API (#5121)

# What this PR does
Truncates text for slack message to avoid this error:
```
File "/usr/local/lib/python3.12/site-packages/slack_sdk/web/slack_response.py", line 199, in validate
    raise e.SlackApiError(message=msg, response=self)
slack_sdk.errors.SlackApiError: The request to the Slack API failed. (url: https://www.slack.com/api/chat.postMessage)
The server responded with: {'ok': False, 'error': 'invalid_blocks', 'errors': ['failed to match all allowed schemas [json-pointer:/blocks/0/text]', 'must be less than 3001 characters [json-pointer:/blocks/0/text/text]'], 'response_metadata': {'messages': ['[ERROR] failed to match all allowed schemas [json-pointer:/blocks/0/text]', '[ERROR] must be less than 3001 characters [json-pointer:/blocks/0/text/text]']}}
```

## Which issue(s) this PR closes

Related to [issue link here]

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## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/apps/slack/models/slack_message.py

@@ -6,6 +6,7 @@ import uuid
 from django.db import models
 
 from apps.slack.client import SlackClient
+from apps.slack.constants import BLOCK_SECTION_TEXT_MAX_SIZE
 from apps.slack.errors import (
     SlackAPIChannelArchivedError,
     SlackAPIError,
@@ -127,6 +128,8 @@ class SlackMessage(models.Model):
         else:
             text = "{}\nInviting {} to look at the alert group.".format(alert_group.long_verbose_name, user_verbal)
 
+        text = text[:BLOCK_SECTION_TEXT_MAX_SIZE]
+
         blocks = [
             {
                 "type": "section",