Fix #12411: [Admin] Send Network Welcome Packet to admin port after game creation completed

[MuxyDuGoulp]

Motivation / Problem

ADMIN_PACKET_SERVER_WELCOME is sent too early when a newgame is issued with an external bot already connected through AdminPort.

See #12411.

Description

Move the sending of the packet to a better appropriate place when the game creation is over : NetworkOnGameStart

Closes #12411.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested') : No
• This PR touches english.txt or translations? Check the guidelines : No
• This PR affects the save game format? (label 'savegame upgrade') : No
• This PR affects the GS/AI API? (label 'needs review: Script API') : No
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF') : No
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[2TallTyler]

Git history looks good now. 😃 Now just leave the Update branch button alone, it's not necessary to keep the PR up-to-date with the base branch as long as the merge rules below say Merging can be performed automatically with 1 approving review.

I am not familiar with AdminPort so I won't review this PR, but I'm sure someone else will take a look soon.