chore: bump APISIX-Runtime to 1.3.18

[nic-6443]

Description

Bumps APISIX_RUNTIME to 1.3.18, with the version guard and the two apisix-runtime-debug checksums in ci/linux-install-openresty.sh.

The only component that moves is ngx_multi_upstream_module 1.3.3 → 1.3.4 (api7/ngx_multi_upstream_module#22), a use-after-free that crashes the worker. ngx_http_multi_upstream_connection_close() destroyed c->pool and then called ngx_close_connection(c), which still logs through c->log — and for these upstream connections c->log lives in the pool that was just released, so ngx_reusable_connection() reads a freed ngx_log_t:

#0  ngx_write_fd (fd=<error reading variable: Cannot access memory ...>)
#1  ngx_log_error_core (fmt="reusable connection: %ui")
#2  ngx_reusable_connection (c=..., reusable=0)
#3  ngx_close_connection (c=...)
#4  ngx_http_multi_upstream_connection_close (c=...)

It surfaces here as an intermittent SIGSEGV in t/plugin/dubbo-proxy/upstream.t TEST 1, which took out the t/plugin/[a-k]*.t shard once during #13863. Running that file five times locally: 2 crashes on 1.3.17, 0 on 1.3.18.

Checksums were taken from the published release artifacts:

0d7cbe27cd0303c6f6b3cad27338a697cf2e2e809ceeac9a57b2d1fb78a3a20e  apisix-runtime-debug_1.3.18-0.debianbookworm-slim_amd64.deb
a7cf5040837e4d34f456e97ff9b858ce64bf95e3b2f647f5d3021de5a9770741  apisix-runtime-debug_1.3.18-0.debianbookworm-slim_arm64.deb

Which issue(s) this PR fixes:

N/A

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

The whole suite runs against the new runtime, which is the coverage a version bump can have; the crash it fixes has no APISIX-side surface to assert on beyond the dubbo test that already exercises it.

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[membphis]

LGTM