RFR: 1625: Validate input from /summary command

[Zhao Song]

On Fri, 21 Oct 2022 21:43:44 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> When quoting the given invalid summary please quote it in the same way as the accepting message below (multiline if needed, use back ticks). Prefix it with "Invalid summary: ". Then on the next line add the information about what cannot be included in a summary. I don't think we need the "This Summary will be removed." message on top of that.
> 
> Example:
> 
> Invalid summary:
> 
> ```
> foobar
> 12345: foo
> ```
> 
> A summary line cannot start with any of the following: `<issue-id>:`, `Co-authored-by:`, `Reviewed-by:`, `Backport-of:`. See [JEP 357](https://openjdk.org/jeps/357) for details.

Thanks for the suggestion! I will change it.

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PR: https://git.openjdk.org/skara/pull/1401