fix(build): reclaim expired prometheus shared dict entries for release/1.23

[wklken]

Description

Backport #170 to release/1.23 (APISIX 3.16.0).

The build patch physically reclaims expired Prometheus metric values and key-index slots from prometheus-metrics after the existing expiry scan, preventing shared-dict slab usage from growing indefinitely under label churn.

Source PR: #170
Upstream fix: api7/nginx-lua-prometheus#18
Related issue: apache/apisix#13658

Cherry-picked commits

• 7b4d193208bc4efa802584eddafacc5a11fef270 fix(build): reclaim expired prometheus shared dict entries

Verification

• Patch dry-run with --fuzz=0 against apache/apisix:3.16.0-redhat
• Built bk-apigateway-apisix:prometheus-flush-release-1.23-codex from this branch
• Verified image label is APISIX 3.16.0
• Verified the built image contains exactly one self.dict:flush_expired() call in prometheus_keys.lua
• LuaJIT bytecode compilation of the patched file succeeded
• RUN_WITH_IT= make lint: 0 warnings / 0 errors
• RUN_WITH_IT= make test: Busted 752 successes; test-nginx 681 tests, all successful

Checklist

• 填写 PR 描述及相关 issue (write PR description and related issue)
• 代码风格检查通过 (code style check passed)
• PR 中包含单元测试 (include unit test) — dependency patch; upstream PR contains regression tests
• 单元测试通过 (unit test passed)
• 本地开发联调环境验证通过 (local development environment verification passed)