[doc] Add gcc version 9+ requirement to getting started
[doc] Add gcc version 9+ requirement to getting started

Edited the gcc-9 requirement instructions in the getting started page, because upgrading the compiler version cannot be done via the default apt repositories on Ubuntu 18.04.
Reference: https://vegastack.com/tutorials/how-to-install-gcc-compiler-on-ubuntu-18-04/
Edited by: Moritz Wettermann <moritz.wettermann@gi-de.com>

Signed-off-by: Marno van der Maas <140765e35f75d33bd28eb112371bcce78847cb8a@lowrisc.org>

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