chore: bump to v5.1.0.alpha.8

apps/emqx/include/emqx_release.hrl

@@ -32,7 +32,7 @@
 %% `apps/emqx/src/bpapi/README.md'
 
 %% Opensource edition
--define(EMQX_RELEASE_CE, "5.1.0-alpha.5").
+-define(EMQX_RELEASE_CE, "5.1.0-alpha.8").
 
 %% Enterprise edition
 -define(EMQX_RELEASE_EE, "5.1.0-alpha.7").