Bump GCC version for Linux builds to 10.3.0

When switching to Firefox 86 (see: #40168 (closed)), compiling our ASan build with GCC 9.3.0 results in

18:56.43 {standard input}: Assembler messages:
18:56.84 {standard input}:2051862: Warning: end of file not at end of a line; newline inserted
18:56.84 {standard input}:2052290: Warning: Unary operator - ignored because bad operand follows
18:56.84 {standard input}:2052290: Error: missing or invalid displacement expression `-'
18:56.84 {standard input}: Error: open CFI at the end of file; missing .cfi_endproc directive
18:57.25 g++.real: fatal error: Killed signal terminated program cc1plus
18:57.25 compilation terminated.
18:57.32 /var/tmp/build/firefox-89324d7a4cef/config/rules.mk:674: recipe for target 'UnifiedBindings22.o' failed
18:57.32 make[4]: *** [UnifiedBindings22.o] Error 1

Bumping GCC to the latest stable solves this issue. (And it is a good thing anyway when doing the switch away from ESR)