Update GCC version used by Centos 7 for CI builds

Centos 7 has GCC 4.8.5 by default. This doesn't provide support for C++14 language features which are likely to be used increasingly within Nektar++ source code going forward.

This issue was initially highlighted by !1343 (merged) and while the code in that MR seems to have been modified to avoid this issue, it would benefit from being resolved to avoid potential future issues - Centos 7 doesn't reach official EOL until 30th June 2024.