build: require flux-core >= 0.49.0

Problem: the pmix shell plugin requires changes that were added
to flux-core 0.49.0 but the minimum version required by configure
is 0.46.0.

flux-pmix is disabled unless specifically enabled by the -opmi=LIST
shell option, but earlier versions of flux-core restrict the values
that this shell option can have so that pmix can never be enabled.

require flux-core >= 0.49.0

config/ax_flux_core.m4

@@ -51,7 +51,7 @@ AC_DEFUN([AX_FLUX_CORE], [
   PKG_CONFIG_PATH=${prefix}/lib/pkgconfig:${PKG_CONFIG_PATH}
   export PKG_CONFIG_PATH
 
-  PKG_CHECK_MODULES([FLUX_CORE], [flux-core >= 0.46.0],
+  PKG_CHECK_MODULES([FLUX_CORE], [flux-core >= 0.49.0],
     [
       FLUX_PREFIX=`pkg-config --variable=prefix flux-core`
       LIBFLUX_VERSION=`pkg-config --modversion flux-core`