Slack channel IDs are now more than 9 letters – adjust the tip

Description

Slack channel ID length grows, so we cannot say "9 letters" any more.

Examples

Old slack ID example: https://gitlab.slack.com/archives/C12345678

New: Old slack ID example: https://gitlab.slack.com/archives/C1234567890

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes and they have been reviewed OR there are no API changes