Improve copy Slack application integration
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## Problem
When user has Slack application integration enabled, the following is shown: (`settings > integrations > slack application`)

* Copy guidelines suggest to not use terminology like `allow` and `enable`
* Copy could be shorter and more concise
Full context in Figma [comment](https://www.figma.com/file/TU7XWkuFMBX1ODckT89ryT?node-id=31:5157#243409220)
## Solution
@ashrafkhamis has suggested to update the copy to the following
```
Perform common actions on this project by entering slash commands in Slack. Learn more.
For a list of available commands, enter '/gitlab help'.
```
issue