chore: fix our devcontainer env to switch to using mage instead of task

[markphelps]

Fixes ability to develop Flipt inside devcontainer https://containers.dev

• removes mention/calling of Task and uses Mage
• updates .devcontainer.json to latest spec
• adds workspaces for server/ui when running in devcontainer (https://code.visualstudio.com/docs/editor/workspaces#_multiroot-workspaces)

[codecov]

Codecov Report

Merging #1468 (cad3f12) into main (4d784ef) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1468   +/-   ##
=======================================
  Coverage   77.77%   77.77%           
=======================================
  Files          44       44           
  Lines        3402     3402           
=======================================
  Hits         2646     2646           
  Misses        603      603           
  Partials      153      153           

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