Run CI over 1.7 rather than 1.8

[chalin]

Currently CI is building and testing over Java 1.8 and 1.11, for example, see https://github.com/grpc/grpc-kotlin/pull/194/checks and the screenshot given below.

Since we're officially endorsing 1.7 as our min supported version of Java, the min Java version should probably be 1.7 in our CI jobs too. (This is partly a followup to #191 and #192.)

cc @davidjwiner

[jamesward]

I tried this in #193 but it didn't work / wasn't easy because gradle requires 1.8 to run. I'm sure there is a way to build an artifact with 1.8 (and have 1.7 compat) but then test the artifact with 1.7. However, before we invest the time we should see how much longer gRPC intends to support 1.7.

[chalin]

However, before we invest the time we should see how much longer gRPC intends to support 1.7

Good idea!

[chalin]

Related: Drop support for Java 7, grpc/grpc-java#4671.