date: Resolve literal white-space deprecation warnings

[Why]
If compiled with clang, the compiler complains:
> include/gul14/date.h:912:41: warning: identifier '_d' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
>   912 | constexpr gul14::date::day  operator "" _d(unsigned long long d) noexcept;
>   |                             ~~~~~~~~~~~~^~
>       |                             operator""_d
> include/gul14/date.h:913:41: warning: identifier '_y' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
>   913 | constexpr gul14::date::year operator "" _y(unsigned long long y) noexcept;
>   |                             ~~~~~~~~~~~~^~
>   |                             operator""_y
> include/gul14/date.h:1813:13: warning: identifier '_d' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
>  1813 | operator "" _d(unsigned long long d) noexcept
>   | ~~~~~~~~~~~~^~
>   | operator""_d
> include/gul14/date.h:1820:13: warning: identifier '_y' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
>  1820 | operator "" _y(unsigned long long y) noexcept
>   | ~~~~~~~~~~~~^~
>       | operator""_y

Signed-off-by: Soeren Grunewald <soeren.grunewald@desy.de>

Author: soerengrunewald

include/gul17/date.h

@@ -908,8 +908,8 @@ operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_weekday_last&
 inline namespace literals
 {
 
-constexpr gul17::date::day  operator "" _d(unsigned long long d) noexcept;
-constexpr gul17::date::year operator "" _y(unsigned long long y) noexcept;
+constexpr gul17::date::day  operator ""_d(unsigned long long d) noexcept;
+constexpr gul17::date::year operator ""_y(unsigned long long y) noexcept;
 
 }  // inline namespace literals
 #endif // !defined(_MSC_VER) || (_MSC_VER >= 1900)
@@ -1809,14 +1809,14 @@ inline namespace literals
 
 constexpr inline
 gul17::date::day
-operator "" _d(unsigned long long d) noexcept
+operator ""_d(unsigned long long d) noexcept
 {
     return gul17::date::day{static_cast<unsigned>(d)};
 }
 
 constexpr inline
 gul17::date::year
-operator "" _y(unsigned long long y) noexcept
+operator ""_y(unsigned long long y) noexcept
 {
     return gul17::date::year(static_cast<int>(y));
 }