[issue tracker] reduced the "days until stale" for issues

Hi,

The issues used to have a stale range of 240 days, I reduced it to 180. This
way people get a reminder before the release for all the issues that they opened early in
the development phase.

After around half a year, the stale bot will put a label on the issue
and add a comment:

  To limit bug bankruptcy (see https://www.joelonsoftware.com/2012/07/09/software-inventory/)
  this issue has been automatically marked as stale because it has not had any activity in 8 months.
  It will be closed in 1 month if no further activity occurs. If this issue remains important to you,
  please comment to reactivate the issue. Thank you for your contributions.


Then 30 days later the issue will be closed if there is no update.

(see for the config of the bot: https://github.com/pharo-project/pharo/blob/Pharo8.0/.github/stale.yml )