Bump ebuilds to EAPI=8

[Totktonada]

https://mgorny.pl/articles/the-ultimate-guide-to-eapi-7.html
https://mgorny.pl/articles/the-ultimate-guide-to-eapi-8.html

Current state:

$ echo '| ebuild | EAPI |'; echo '| ---- | ---- |'; grep -r EAPI . | tr ':' ' ' | sed s@^./@@ | sort -k2,2 | awk '{ print "| "$1" | "$2" |"}'

ebuild	EAPI
www-servers/nginx/nginx-1.17.8.ebuild	EAPI="6"
dev-db/tarantool/tarantool-9999.ebuild	EAPI=6
dev-libs/msgpuck/msgpuck-9999.ebuild	EAPI=6
dev-lua/avro-schema/avro-schema-9999.ebuild	EAPI=6
dev-lua/checks/checks-9999.ebuild	EAPI=6
dev-lua/connpool/connpool-9999.ebuild	EAPI=6
dev-lua/expirationd/expirationd-9999.ebuild	EAPI=6
dev-lua/http/http-9999.ebuild	EAPI=6
dev-lua/mysql/mysql-9999.ebuild	EAPI=6
dev-lua/queue/queue-9999.ebuild	EAPI=6
dev-lua/shard/shard-9999.ebuild	EAPI=6
dev-lua/smtp/smtp-9999.ebuild	EAPI=6
dev-libs/tarantool-c/tarantool-c-9999.ebuild	EAPI=7
dev-lua/argon2/argon2-9999.ebuild	EAPI=7
dev-lua/gperftools/gperftools-9999.ebuild	EAPI=7
dev-lua/luatest/luatest-9999.ebuild	EAPI=7
dev-lua/pg/pg-9999.ebuild	EAPI=7

[Totktonada]

NB: Ebuilds will be updated to EAPI=7 in the scope of #50.