Mark NETMSGTYPE_CL_STARTINFO as MSGFLAG_FLUSH

[Robyt3]

This makes the client connect to the server faster. It's especially noticeable on servers with a low ping.

Closes #6811.

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test (especially base/) or added coverage to integration test
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with ASan+UBSan or valgrind's memcheck (optional)

[def-]

bors r+

[bors]

Build succeeded!

The publicly hosted instance of bors-ng is deprecated and will go away soon.

If you want to self-host your own instance, instructions are here.
For more help, visit the forum.

If you want to switch to GitHub's built-in merge queue, visit their help page.

• build-cmake (macOS-latest)
• build-cmake (ubuntu-20.04)
• build-cmake (ubuntu-latest)
• build-cmake (windows-latest)
• check-clang-san
• check-clang-tidy
• check-style