Drop support for Go 1.21

[marefr]

What would you like to be added:
Drop support for Go 1.21, suggested at least Go 1.22.

Why is this needed:
To allow otel dependency updates (which dropped support for Go 1.21), see #1092, #1086, #1084 and #1083

Additional info:
Probably need to update https://github.com/grafana/plugin-tools/blob/77727ec5a8ac665fdc71dc7bf97b60650713e884/packages/create-plugin/templates/backend/go.mod#L3 and https://github.com/grafana/grafana-plugin-examples as well