Simplify fs::path by dropping filename() and make_preferred() overloads

These overloads were needed to allow passing `fs::path` objects directly to
libstdc++'s `fstream` constructors, but after the previous commit, there is no
longer any remaining code that does pass `fs::path` objects to `fstream`
constructors. Writing new code which does this is also discouraged because the
standard has been updated in https://wg21.link/lwg3430 to disallow it.

Dropping these also means its no longer possible to pass `fs::path` arguments
directly to `fstream::open` in libstdc++, which is somewhat unfortunate but not
a big loss because it is already not possible to pass them to the constructor.
So this commit updates `fstream::open` calls.

Additionally, this change required updates to src/bitcoin.cpp since it was
relying on the overloaded filename() method.

src/rpc/request.cpp

@@ -114,7 +114,7 @@ GenerateAuthCookieResult GenerateAuthCookie(const std::optional<fs::perms>& cook
     if (filepath_tmp.empty()) {
         return GenerateAuthCookieResult::DISABLED; // -norpccookiefile
     }
-    file.open(filepath_tmp);
+    file.open(filepath_tmp.std_path());
     if (!file.is_open()) {
         LogWarning("Unable to open cookie authentication file %s for writing", fs::PathToString(filepath_tmp));
         return GenerateAuthCookieResult::ERR;
@@ -153,7 +153,7 @@ bool GetAuthCookie(std::string *cookie_out)
     if (filepath.empty()) {
         return true; // -norpccookiefile
     }
-    file.open(filepath);
+    file.open(filepath.std_path());
     if (!file.is_open())
         return false;
     std::getline(file, cookie);