docs: reporting-issues: Remove reference to oldnoconfig

Replace it with olddefconfig. oldnoconfig didn't do what the document
suggests (it aliased to olddefconfig), and isn't available since 4.19.

Ref: 04c459d2 ("kconfig: remove oldnoconfig target")
Ref: 312ee687 ("kconfig: announce removal of oldnoconfig if used")
Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
Link: https://lore.kernel.org/r/20210331163541.28356-1-ismael@iodev.co.uk


Signed-off-by: Jonathan Corbet <corbet@lwn.net>