rpm: bump up required GCC version to 7.3.1

rpm: bump up required GCC version to 7.3.1

so we can include the fix of
https://gcc.gnu.org/ml/libstdc++/2017-07/msg00077.html and the fix at
https://github.com/gcc-mirror/gcc/commit/7c78f2e67270187dd755632eafe3a526f4e6df30
. otherwise, if a class offers implicit conversion operator to
std::string and std::string_view, seastar will fail to compile. like

/opt/rh/devtoolset-7/root/usr/include/c++/7/bits/basic_string.h:3946:7:
note: candidate: std::basic_string<_CharT, _Traits, _Alloc>&
std::basic_string<_CharT, _Traits, _Alloc>::operator+=(const
std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char;
_Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       operator+=(const basic_string& __str)
       ^~~~~~~~
/opt/rh/devtoolset-7/root/usr/include/c++/7/bits/basic_string.h:3988:7:
note: candidate: std::basic_string<_CharT, _Traits, _Alloc>&
std::basic_string<_CharT, _Traits,
_Alloc>::operator+=(std::basic_string<_CharT, _Traits,
_Alloc>::__sv_type) [with _CharT = char; _Traits =
std::char_traits<char>; _Alloc = std::allocator<char>;
std::basic_string<_CharT, _Traits, _Alloc>::__sv_type =
std::basic_string_view<char>]
       operator+=(__sv_type __sv)
       ^~~~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>