MINOR: Upgrade to Gradle 4.8.1 (#5263)
MINOR: Upgrade to Gradle 4.8.1 (#5263)

Maven Central dropped support for all versions
but TLS 1.2, so dependency resolution fails if
Gradle builds run with JDK 7. 2.0 and trunk
requre JDK 8, but every other version is
affected.

Reviewers: Rajini Sivaram <2e743c5cf650e12d7db950775eab7636545b5b8c@googlemail.com>
richhaase/kafka,richhaase/kafka,apache/kafka,richhaase/kafka,gf53520/kafka,Chasego/kafka,ollie314/kafka,KevinLiLu/kafka,richhaase/kafka,mihbor/kafka,Esquive/kafka,guozhangwang/kafka,TiVo/kafka,Chasego/kafka,guozhangwang/kafka,lindong28/kafka,guozhangwang/kafka,mihbor/kafka,noslowerdna/kafka,noslowerdna/kafka,noslowerdna/kafka,apache/kafka,sslavic/kafka,lindong28/kafka,KevinLiLu/kafka,Esquive/kafka,sslavic/kafka,noslowerdna/kafka,sslavic/kafka,lindong28/kafka,Esquive/kafka,guozhangwang/kafka,apache/kafka,Chasego/kafka,lindong28/kafka,TiVo/kafka,mihbor/kafka,Esquive/kafka,gf53520/kafka,ollie314/kafka,sslavic/kafka,KevinLiLu/kafka,apache/kafka,gf53520/kafka,ollie314/kafka,ollie314/kafka,Chasego/kafka,TiVo/kafka,TiVo/kafka,mihbor/kafka,gf53520/kafka,KevinLiLu/kafka