Add: [Dependabot] introduce Dependabot to keep our workflows up-to-date

[TrueBrain]

Motivation / Problem

Updating workflows is a very time-intensive task, where you have to look up every action, see if they have a new version, etc. There is a better way: DEPENDABOT!

Description

Install dependabot for workflows.

Limitations

Might trigger some people as every month it will make a PR. The first person to bitch about it can maintain the workflow after that. Deal?

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')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • 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')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker