support building with GCC 13 & drop old OSs

[kfix]

tested with CC=gcc-13 CXX=g++-13 ./build.sh

fixes errors like:

In file included from src/srvr/access.cc:13:
include/is2/srvr/access.h:41:9: error: 'uint8_t' does not name a type
   41 |         uint8_t m_rqst_http_major;
      |         ^~~~~~~
include/is2/srvr/access.h:19:1: note: 'uint8_t' is defined in header '<cstdint>'; did you forget to '#include <cstdint>'?
   18 | #include <string>
  +++ |+#include <cstdint>
   19 | namespace ns_is2 {

[airencracken]

lgtm

[airencracken]

still

L G T M