[backport -> release/3.6.x] fix(vault): let vault entity cache key not containing workspace id

[team-gateway-bot]

Automated backport to release/3.6.x, triggered by a label in #13610.

Original description

Summary

This PR modifies the cache_key function of the vault entity to always generate a cache key without workspace id.

Vault entity is workspace-able, but our secret rotation timer always run without workspace settings(thus the default workspace is being used), so during secret rotation, the code

kong/kong/pdk/vault.lua

Lines 620 to 621 in 4e38b96

	local vault_cache_key = vaults:cache_key(prefix)
	vault, err = cache:get(vault_cache_key, nil, vaults.select_by_prefix, vaults, prefix, VAULT_QUERY_OPTS)

will generate a duplicate vault cache with default workspace id for each non-default workspace vault entity, and those cache will never be refreshed. The result of this issue is that when you update a vault entity's configuration inside a non-default workspace, it will never take effect in the secret rotation.

Since the prefix of vault entity is unique across workspaces, it should be safe to only use one cache key without workspace id, so that the correct cache is used during secret rotation.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

FTI-6152