chore: leftovers for centralized OTLPGateway refactoring

[TeodorSAP]

Description

Changes proposed in this pull request (what was done and why):

• Adjust load tests to new OTLPGateway architecture
• Add legacy resources cleanup logic
• Deprecate obsolete gateway scaling feature
• Adapt documentation to new architecture + istio mTLS sec. advisory
• Add node watcher logic to the OTLP Gateway controller
• Standardize controllers' code

Changes refer to particular issues, PRs or documents:

• Autoscaling for telemetry gateways #424

Traceability

• The PR is linked to a GitHub issue.
• The follow-up issues (if any) are linked in the Related Issues section.
• If the change is user-facing, the documentation has been adjusted.
• If a CRD is changed, the corresponding Busola ConfigMap has been adjusted.
• The feature is unit-tested.
• The feature is e2e-tested.