[3.1] libapparmor: fix mistaken SO version bump

In commit 7c7224004 ("Prepare for AppArmor 3.1 release"), as preperation
for the AppArmor 3.1.0 release, the SO versioning information was
adjusted, using a more significant bump to give prior AppArmor releases
room to address bugs in libapparmor without ending up with conflicting
SO versions. Unfortunately, that process was untested and because
AA_LIB_AGE was not incremented by the same amount as AA_LIB_CURRENT,
this resulted in an accidental major SO versions bump with the library
SO version being:

  libapparmor.so.4.9.0

This commit increments AA_LIB_AGE by the same amount, resulting in a
library versioned as:

  libapparmor.so.1.12.0

and adds a note to mention that AA_LIB_AGE needs to be incremented
in the same way as AA_LIB_CURRENT. This fix is intended to address
this for the 3.1 branch; I'd like to find a better approach for
the development branch that can be used in future AppArmor primary
releases.

In general, thanks to symbol versioning (see
`libraries/libapparmor/src/libapparmor.map`) we should not need to
ever bump the SO version except in an extreme case.

Fixes: 7c7224004 ("Prepare for AppArmor 3.1 release")
Signed-off-by: Steve Beattie <steve.beattie@canonical.com>
Bug: https://gitlab.com/apparmor/apparmor/-/issues/266

libraries/libapparmor/src/Makefile.am

@@ -16,7 +16,8 @@ INCLUDES = $(all_includes)
 #        problems in the unlikely event where an interface has to break.
 #    - set AA_LIB_REVISION to 0.
 # 4. If any interfaces have been added since the last public release, then
-#    - increment AA_LIB_AGE.
+#    - increment AA_LIB_AGE by the same amount that AA_LIB_CURRENT was
+#      incremented.
 # 5. If any interfaces have been removed or changed since the last public
 #    release, then
 #    - set AA_LIB_AGE to 0.
@@ -31,7 +32,7 @@ INCLUDES = $(all_includes)
 #
 AA_LIB_CURRENT = 13
 AA_LIB_REVISION = 0
-AA_LIB_AGE = 9
+AA_LIB_AGE = 12
 
 SUFFIXES = .pc.in .pc