Issue template should include link to old repo issues too

[kkmuffme]

The issue template currently contains:

• I have searched the issue list and am not opening a duplicate issue.

I suggest to change this to:

• I have searched the issue list here and in https://github.com/squizlabs/PHP_CodeSniffer/issues/ and am not opening a duplicate issue.

Since many issues were often already discussed previously. This will not only reduce the number of new tickets but also allow users to link any existing issues to their new issue, in case they still want to go ahead to open an issue, since it's something that's still relevant.

[jrfnl]

While issues in the Squizlabs repo may still be addressed, it is not a list actively monitored anymore, so opening an issue which was previously discussed in the Squizlabs repo is perfectly fine.

It helps if links to the previous issues are added, but those can also be added later when someone triages an issue, so no, I'd rather not give people the impression that the Squizlabs repo is still active.