Update nektar submodule to v5.6.0.

[oparry-ukaea]

Description

Update nektar submodule to v5.6.0. No changes to NESO code required.

Type of change

• [x] New feature (non-breaking change which adds functionality)

Testing

No new tests. Passes existing unit and integration tests.

Test Configuration:

OS: Ubuntu 22.04 Compiler: GCC 11.3.0 / OneAPI v2022.1.0 SYCL implementation: Hipsycl v0.9.2 / DPC++ v2022.1.0 MPI details: MPICH v4.0.2 Hardware: CPU (Intel Alder Lake)

Checklist:

• [x] New and existing unit tests pass locally with my changes
• [x] My changes generate no new warnings ~- [ ] I have added tests that prove my fix is effective or that my feature works~ ~- [ ] I have used understandable variable names~ ~- [ ] I have run clang-format against my *.hpp and *.cpp changes~ ~- [ ] I have made corresponding changes to the documentation~ ~- [ ] I have performed a self-review of my own code~ ~- [ ] I have commented my code, particularly in hard-to-understand areas~

[oparry-ukaea]

No pressing need for this, but I think it's useful to keep nektar up-to-date if we can.

[will-saunders-ukaea]

Unit and integrations tests passed for me with adaptivecpp.