Change GCC version to 8.3.0 for Ubuntu 18.04

We need to use gcc 8 instead of 9 so that the Qt installer
will work with CentOS 8.3 which have gcc 8 in the system.
If we use newer gcc version with the binaries there will be issues
with libstdc++.
Using gcc-toolset-9 in CentOS won't help either because it will link
libstdc++ against the one in system.

Task-number: QTBUG-91352
Change-Id: I2a3d186bc04b1f54f14ed95f0588d18bd63814c4
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>
(cherry picked from commit 8843f51866)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

coin/provisioning/common/linux/gcc.sh

@@ -2,7 +2,7 @@
 
 #############################################################################
 ##
-## Copyright (C) 2020 The Qt Company Ltd.
+## Copyright (C) 2021 The Qt Company Ltd.
 ## Contact: http://www.qt.io/licensing/
 ##
 ## This file is part of the provisioning scripts of the Qt Toolkit.
@@ -57,7 +57,7 @@ function InstallGCC() {
     suffixVersion=$(echo "$version" | cut -d "." -f1,2)
     sourceFile="gcc-$version.tar.xz"
     cachedUrl="http://ci-files01-hki.intra.qt.io/input/gcc/$sourceFile"
-    officialUrl="ftp://ftp.mpi-sb.mpg.de/pub/gnu/mirror/gcc.gnu.org/pub/gcc/releases/gcc-$version/$sourceFile"
+    officialUrl="https://gcc.gnu.org/pub/gcc/releases/gcc-$version/gcc-$version.tar.xz"
 
     targetFile="$tmpFolder/$sourceFile"
     buildFolder="$HOME/gcc_build"

coin/provisioning/qtci-linux-Ubuntu-18.04-x86_64/03-gcc.sh

@@ -5,4 +5,4 @@ set -ex
 # shellcheck source=../common/linux/gcc.sh
 source "${BASH_SOURCE%/*}/../common/linux/gcc.sh"
 
-InstallGCC 9.3.0 50 5038e8752407d14e5a70c8efc80c20a6d4219aaa 212f77d7b7fe1fdf01a1c0b0ebc9d82aeda5e1e0
+InstallGCC 8.3.0 50 ccccfe4fe9206d111a173c19a21f8700d1133ae8 c27f4499dd263fe4fb01bcc5565917f3698583b2