Drop support for Java 7

[andreamlin]

Clients won't be able to fetch dependencies from Maven due to Java 7 not supporting TLS 1.2 by default
Sample failed CI build, note the "Received fatal alert: protocol_version" message:

[ERROR] Failed to execute goal on project proto-google-cloud-bigquerydatatransfer-v1:
 Could not resolve dependencies for project com.google.api.grpc:proto-google-cloud-bigquerydatatransfer-v1:jar:0.18.1-SNAPSHOT:
 Failed to collect dependencies at com.google.api.grpc:proto-google-common-protos:jar:1.12.0:
 Failed to read artifact descriptor for com.google.api.grpc:proto-google-common-protos:jar:1.12.0:
 Could not transfer artifact com.google.api.grpc:proto-google-common-protos:pom:1.12.0 from/to central (https://repo.maven.apache.org/maven2):
 Received fatal alert: protocol_version -> [Help 1]

This will continue to break any CI builds that use Java 7, e.g. in google-cloud-java.

This can be mitigated by #3427, which will solve the dependency fetch issue, but still leaves open concerns that we are supported an outdated, potentially insecure version of Java.

Java 7 doesn't have public security updates any more, and it has been deprecated by Oracle.

[garrettjonesgoogle]

But what about https://tonyyan.wordpress.com/2015/07/17/enabled-tls-1-2-and-tls-1-1-on-java-7/ ?

[andreamlin]

I've spent a couple days trying to apply fixes to enable TLS 1.2 on Java 7 without any luck, but if someone else can do it that would be great!

[andreamlin]

Also even if we enable TLS 1.2 on Java 7, users would have to do the same thing.

[andreamlin]

We should get sign-off from the DPE, LOL, and Java gRPC teams.

[andreamlin]

Apiary libraries have yet to turn down Java 5 support, so this issue won't be resolved for a long time. Closing for now.