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

Documentation/admin-guide/reporting-issues.rst

@@ -1067,8 +1067,7 @@ In the whole process keep in mind: an issue only qualifies as regression if the
 older and the newer kernel got built with a similar configuration. The best way
 to archive this: copy the configuration file (``.config``) from the old working
 kernel freshly to each newer kernel version you try. Afterwards run ``make
-oldnoconfig`` to adjust it for the needs of the new version without enabling
-any new feature, as those are allowed to cause regressions.
+olddefconfig`` to adjust it for the needs of the new version.
 
 
 Write and send the report