supported-platforms: correct supported GCC version on Ubuntu 16.04

A default compiler version on Ubuntu is GCC 5.4.0, not GCC 6 as
was set in 73c29d33

. WMs in CI use
the default version.

gcc-6 --version
The program 'gcc-6' is currently not installed. You can install it by typing:
sudo apt install gccgo-6

Change-Id: I9029f81bd9ea7c06142403b0bd7dd7035dd80fcc
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>

doc/src/platforms/supported-platforms.qdocinc

@@ -80,7 +80,7 @@ Mobile Platforms: \l {Qt for Android}{Android},
          \li \b {GCC 5.3.1}
          \li devtoolset-4
     \row \li Ubuntu 16.04 (x86_64)
-         \li GCC as provided by Canonical, \b {GCC 6}
+         \li GCC as provided by Canonical, \b {GCC 5.4.0}
          \li
     \row \li Linux (x86 and x86_64)
          \li GCC 4.8, GCC 4.9, GCC 5, GCC 6, GCC 7
@@ -167,7 +167,7 @@ Mobile Platforms: \l {Qt for Android}{Android},
          \li \b {GCC 5.3.1}
          \li devtoolset-4
     \row \li Ubuntu 16.04 (x86_64)
-         \li GCC as provided by Canonical, \b {GCC 6}
+         \li GCC as provided by Canonical, \b {GCC 5.4.0}
          \li
     \row \li Linux (x86 and x86_64)
          \li GCC 4.8, GCC 4.9, GCC 5, GCC 6, GCC 7