fix(plugins): add old_default to redis shorthand fields to prevent spurious deprecation warnings

[alexezio]

Summary

This PR fixes an issue where rate-limiting and response-ratelimiting plugins were generating excessive deprecation warnings even when users never configured any redis-related parameters.

Root Cause Analysis:

1. When reading plugin configuration, Kong reverse-maps the new redis.* field values back to deprecated redis_* shorthand fields (for backwards compatibility)
2. The redis.* fields have default values (e.g., ssl=false, port=6379, timeout=2000, database=0)
3. These default values get reverse-mapped to redis_* fields on every config read
4. Since the redis_* shorthand field definitions lacked old_default values, every value (including defaults) triggered deprecation warnings
5. According to kong/db/schema/metaschema.lua:
   • If old_default is not set, the warning is always printed
   • If old_default is set, the warning is only printed when the value differs from old_default

The Fix:

Added old_default values to shorthand fields that have corresponding defaults in redis.*:

Field	old_default
redis_port	6379
redis_ssl	false
redis_ssl_verify	false
redis_timeout	2000
redis_database	0

Fields without defaults (redis_host, redis_password, redis_username, redis_server_name) don't need old_default as nil values don't trigger the reverse mapping.

Before this fix:

[kong] init.lua:904 rate-limiting: config.redis_ssl is deprecated, please use config.redis.ssl instead

(Logged on every request, even when user never configured redis)

After this fix:

• No deprecation warnings when using default values
• Warnings still appear when users explicitly use deprecated redis_* config with non-default values

Related Issue

Fixes #14418

Test Plan

• Existing integration tests in spec/03-plugins/23-rate-limiting/05-integration_spec.lua and spec/03-plugins/24-response-rate-limiting/05-integration_spec.lua verify:
  • Using new redis.* config format → no deprecation warnings
  • Using legacy redis_* config with non-default values → deprecation warnings appear
• Manual verification: Create rate-limiting plugin without any redis config, confirm no deprecation warnings in logs

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