docs: set version to 1.8.1 #714

Job	Run time
Build Sablier docker image once and share it to E2E jobs	1m 33s
Build ProxyWasm Plugin once and share it to ProxyWasm E2E jobs	31s
Build Caddy docker image once and share it to Caddy E2E jobs	1m 31s
Build Caddy Sablier Plugin	23s
Build Traefik Sablier Plugin	13s
Run Sablier E2E tests for Nginx NJS module with Sablier (docker)	1m 5s
Run Sablier E2E tests for Traefik middleware (docker)	1m 16s
Run Sablier E2E tests for Nginx NJS module with Sablier (docker_swarm)	2m 20s
Run Sablier E2E tests for Proxywasm middleware on Apache APISIX (docker)	1m 21s
Run Sablier E2E tests for Traefik middleware (docker_swarm)	2m 27s
Run Sablier E2E tests for Caddy middleware (docker)	1m 8s
Run Sablier E2E tests for Traefik middleware (kubernetes)	1m 54s
Run Sablier E2E tests for Caddy middleware (docker_swarm)	2m 52s
Run Sablier E2E tests for Proxywasm middleware on Envoy (docker)	1m 21s
Run Sablier E2E tests for Proxywasm middleware on Nginx (docker)	1m 23s
	21m 18s

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