techdebt: drop _LINUX_CAPABILITY_VERSION_2 support

In starter we are currently allowing _LINUX_CAPABILITY_VERSION_2, but
this was specific to kernel 2.6.25, and deprecated in 2.6.26, which is
older than used by any system we target / test on.

See also:

https://github.com/torvalds/linux/blob/v2.6.26/include/linux/capability.h#L34

https://man7.org/linux/man-pages/man2/capset.2.html

 Kernels prior to 2.6.25 prefer 32-bit capabilities with version
 _LINUX_CAPABILITY_VERSION_1.  Linux 2.6.25 added 64-bit
 capability sets, with version _LINUX_CAPABILITY_VERSION_2.  There
 was, however, an API glitch, and Linux 2.6.26 added
 _LINUX_CAPABILITY_VERSION_3 to fix the problem.

Author: dtrudg

cmd/starter/c/include/capability.h

@@ -19,11 +19,9 @@
 /* 37 is the latest cap since many kernel versions */
 #define CAPSET_MAX  37
 
-/* Support only 64 bits sets, since kernel 2.6.25 */
+/* Support only 64 bits sets, since kernel 2.6.26 */
 #ifdef _LINUX_CAPABILITY_VERSION_3
 #  define LINUX_CAPABILITY_VERSION  _LINUX_CAPABILITY_VERSION_3
-#elif defined(_LINUX_CAPABILITY_VERSION_2)
-#  define LINUX_CAPABILITY_VERSION  _LINUX_CAPABILITY_VERSION_2
 #else
 #  error Linux 64 bits capability set not supported
 #endif /* _LINUX_CAPABILITY_VERSION_3 */