fix: fix listing oncall users when more than 1 page

[brojd]

Related to https://github.com/grafana/support-escalations/issues/14634

When public API of OnCall is hit from the TF provider, we always pass page=1. In the datasource code of the provider there's a loop that depends on the presence of next in paginated response. And it seems we don't bump the page so the next is always there causing infinite chain of requests

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[Duologic]

Approved but know that @grafana/platform-monitoring does not do gatekeeping, teams should feel empowered to get things merged on their own.