Document stack upgrades for development workspaces

[csandanov]

The workspace page said a development workspace cannot upgrade its stack. Workspaces can now be upgraded, so the page explains how.

• New section "Upgrade the stack" on the development workspaces page: what an upgrade does to a workspace (code services stop, the environment is deployed, setup runs again, the SSH runner restarts), that code and the home directory are kept, when the upgrade is refused, and that auto-upgrade is available and off by default.
• The stack page's Upgrade section links to it.
• The limit that listed stack upgrades as unsupported is removed.

Draft until the feature is available.

Validation

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