[doc] remove the supportng of el6

As we discussed in mail list [1] no longer to
support CentOS 6/RHEL 6, Ubuntu 14 and etc,
this patch clarify this in some more docs and
scripts.

1. https://lists.apache.org/thread/7sjcj4ndpz451yj91zmv9538gs791j5q

Change-Id: Idd0385adcbbdee99fd6a5c3b84b86376d9e5ade7
Reviewed-on: http://gerrit.cloudera.org:8080/20146
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <[email protected]>

RELEASING.adoc

@@ -268,8 +268,8 @@ NOTE: If `uploadArchives` is executed without `--no-parallel`, uploading a
 number of artifacts fails with "peer not authenticated" errors.
 
 . Build and deploy new binary test JARs for the RC on macOS and Linux. Build
-  the Linux JAR on a CentOS 6.6 image, and build the macOS JAR on macOS
-  Yosemite if possible (see
+  the Linux JAR on a CentOS 7 image, and build the macOS JAR on macOS
+  High Sierra if possible (see
   link:https://issues.apache.org/jira/browse/KUDU-2724[KUDU-2724] to remove the
   need to build on an old version of macOS).
 +

build-support/mini-cluster/LICENSE-BINARY-JAR-LINUX.txt

@@ -8,12 +8,12 @@ distributed with the Linux version of the Kudu binary test jar.
 --------------------------------------------------------------------------------
 OpenSSL: OpenSSL software license
 libraries: libssl, libcrypto
-Source: Centos 6.6 yum repository
+Source: Centos 7 yum repository
 
 --------------------------------------------------------------------------------
 MIT Kerberos Version 5: MIT license
 libraries: libgssapi_krb5, libkrb5, libkrb5support, libk5crypto
-Source: Centos 6.6 yum repository
+Source: Centos 7 yum repository
 
   Copyright (C) 1985-2013 by the Massachusetts Institute of Technology.
 
@@ -86,7 +86,7 @@ Source: http://curl.haxx.se/
 Cyrus SASL: BSD license
 libraries: libsasl2, libanonymous, libgssapiv2, liblogin, libplain, libsasldb
 libraries: libgs2, libdigestmd5, libcrammd5, libscram, libntlm, libpcre2
-Source: Centos 6.6 yum repository
+Source: Centos 7 yum repository
 
   Copyright (c) 1998-2003 Carnegie Mellon University.  All rights reserved.
 
@@ -154,7 +154,7 @@ Source: yaml-cpp-yaml-cpp-0.6.2 source tarball
 --------------------------------------------------------------------------------
 NSS Softokn Cryptographic Module Freebl Library: MPL 2.0 license
 libraries: libfreebl3
-Source: Centos 6.6 yum repository
+Source: Centos 7 yum repository
 
   NSS is available under the Mozilla Public License, version 2, a copy of which
   is below.

docker/docker-build.py

@@ -197,7 +197,7 @@ def get_full_tag(repository, target, version_tag, os_tag):
       Examples:
         get_tag "kudu" "latest" ""        = apache/kudu:latest
         get_tag "base" "1.8.0" ""         = apache/kudu:base-1.8.0
-        get_tag "base" "1.8.0" "centos6"  = apache/kudu:base-1.8.0-centos6
+        get_tag "base" "1.8.0" "centos7"  = apache/kudu:base-1.8.0-centos7
   """
   full_tag = ''
   # Only include the target if this isn't the default.

docs/developing.adoc

@@ -328,8 +328,8 @@ The binary test JAR must be run on one of the
 <<installation.adoc#prerequisites_and_requirements,supported Kudu platforms>>,
 which includes:
 
-- macOS El Capitan (10.11) or later;
-- CentOS 6.6+, Ubuntu 14.04+, or another recent distribution of Linux
+- macOS High Sierra (10.13) or later;
+- CentOS 7+, Ubuntu 18.04+, or another recent distribution of Linux
 
 The related Maven integration using `os-maven-plugin` requires Maven 3.1 or later.