fix(notifications): Remove deprecated retryingSink with backoffSink and add backward compatibility

What does this MR do?

This MR completes the migration from the deprecated retryingSink to the backoffSink for handling notification retries, while maintaining full backward compatibility for existing configurations using the threshold parameter.

It removes retryingSink that was using fixed backoff periods with unlimited retries which by itself could lead to head-of-the-queue blocking and either container registry resource exhaustion or dropped events. Instead, it standardizes on backoffSink - all notification retries now use exponential backoff with configurable max retries, providing more predictable and efficient retry behavior.

In order to avoid breaking change we translate the values of retryingSink in translateBackoffParams() function to equivalent maxretries values.

Translation logic:

• Calculates a time window of 120 * backoff_duration seconds (based on GitLab's production configuration)
• Simulates exponential backoff to determine how many retries fit within this window
• Ensures the translated value is never less than the original threshold to maintain minimum retry guarantees

I am marking this as a fix because unlimited retries in the old retryingSink can lead to stability issues, and the configuration translation, even though it is a small change in behaviour, is actually a fix that improves container-registry stability and resilience. Customers willing to regain full control of notifications retries should update their configuration to use maxretries at their conveniance. Those that do not need it can stay with defaults/translation behaviour which should be fine for most cases.

Related to #1244 (closed)

Related to #1243 (closed)

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