fix(ske-observability): remove read-only continue from alert route

[dieg666]

route.continue is a read-only (computed) attribute of stackit_observability_instance; setting it broke terraform validate with Invalid Configuration for Read-Only Attribute. Affects both observability examples.

Description

route.continue is a Read-Only (computed) attribute of stackit_observability_instance,
but both observability examples set continue = true in local.alert_config.route, so
they fail terraform validate:

Error: Invalid Configuration for Read-Only Attribute
  with stackit_observability_instance.example
  Cannot set value for this attribute as the provider has marked it as read-only.

Removes the offending line from:

• examples/ske-observability-log-alerts/main.tf
• examples/ske-observability-alerting-kube-state-metrics/main.tf

continue is only meaningful on child route.routes[], not the root route, so dropping
it is also semantically correct.

Ref: provider docs for stackit_observability_instance
(route nested schema → continue listed under Read-Only).

Checklist

• The CI pipeline passed successfully.

[h3adex]

Ty!