fix: make GetWorkspacesEligibleForTransition return even less false positives (#15594)

Relates to https://github.com/coder/coder/issues/15082

Further to https://github.com/coder/coder/pull/15429, this reduces the
amount of false-positives returned by the 'is eligible for autostart'
part of the query. We achieve this by calculating the 'next start at'
time of the workspace, storing it in the database, and using it in our
`GetWorkspacesEligibleForTransition` query.

The prior implementation of the 'is eligible for autostart' query would
return _all_ workspaces that at some point in the future _might_ be
eligible for autostart. This now ensures we only return workspaces that
_should_ be eligible for autostart.

We also now pass `currentTick` instead of `t` to the
`GetWorkspacesEligibleForTransition` query as otherwise we'll have one
round of workspaces that are skipped by `isEligibleForTransition` due to
`currentTick` being a truncated version of `t`.

coderd/apidoc/docs.go

@@ -14543,6 +14543,10 @@ const docTemplate = `{
                 "name": {
                     "type": "string"
                 },
+                "next_start_at": {
+                    "type": "string",
+                    "format": "date-time"
+                },
                 "organization_id": {
                     "type": "string",
                     "format": "uuid"