fix: downgrade decrypt failure log from warn to info

[nic-6443]

During upgrades, when new fields are added to encrypt_fields, existing plaintext data in etcd will fail to decrypt. This is a normal and expected scenario — the plaintext value is preserved as-is and still works. However, the warn-level log generates noise that may alarm users.

This change:

• Downgrades decrypt failure log from warn to info
• Keeps encrypt failure log at warn (those are unexpected)
• Adds a hint message explaining the likely cause (upgrade) and resolution (re-save via Admin API)

Before:

[warn] failed to decrypt the conf of plugin [basic-auth] key [password], err: decrypt ssl key failed

After:

[info] failed to decrypt the conf of plugin [basic-auth] key [password], err: decrypt ssl key failed. This is expected after upgrading if the field was recently added to encrypt_fields; re-save the configuration via the Admin API to resolve

[Copilot]

Pull request overview

Adjusts APISIX plugin configuration encryption/decryption logging to reduce expected log noise during upgrades when encrypt_fields changes, while still highlighting unexpected encryption failures.

Changes:

• Downgrades decrypt failure logs from warn to info.
• Appends an upgrade/resolution hint to decrypt failure logs.
• Keeps encrypt failure logs at warn.

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[nic-6443]

Addressed in de7b7c8 — updated the hint wording to use "This can happen after upgrading..." instead of "This is expected..." and added a note about verifying the data_encryption keyring for genuine decryption failures.