github: reclassify generic issue report to bug_report

This will allow to use multiple GitHub issue templates.
Each bug report will be assigned "Sighting" label.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: I2285473316fce81ee79f28818020ab28e990b7b9
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1598


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>