Release v1.2.3.9 (#3998)

Changelog:
- [P4Testgen] Clean up the implementation of the BMv2 clone externs (3976) [Fabian Ruffy]
- Split state variables and symbolic variables. Stricter type checking. (3987) [Fabian Ruffy]
- Run linters across test folder. (3982) [Fabian Ruffy]
- Add some more type information to the strength reduction pass. (3993) [Fabian Ruffy]
- FreeBSD patches (3991) [Jason Bacon]
- Also include the backend IR files in the ir-generated library file. (3985) [Fabian Ruffy]
- Set up P4C for IWYU (3981) [Fabian Ruffy]
- Make StateVariable an IR class that accepts IR::Member. Use ICastable for some nodes. (3741) [Fabian Ruffy]
- Cleanup some P4Testgen code.  (3978) [Fabian Ruffy]
- Bump Bazel dependency versions to enable using latest Bazel (3979) [Bili Dong]
- Add style and formating files for Python. Format Python code. (3870) [Fabian Ruffy]
- Implement support for default initializers (...) (3968) [Mihai Budiu]
- Always insert instantiation statement at the end of the local statement list (3962) [Bili Dong]
- ir generator fixes + cleanup (3975) [Chris Dodd]
- initialize json pointer (coverity fix) (3972) [Glen Gibb]
- ebpf/PSA: Fix weekly PTF tests (3969) [Jan Palimąka]
- Resolve types in the prepend internal extern. (3964) [Fabian Ruffy]
- Change array index[] in table keys in context.json to $ to match with the key name in bfrt.json (3963) [Usha Gupta]
- Clean up the trace event class of P4Tools.  (3950) [Fabian Ruffy]
- Remove gsl-lite module. Introduce std::referencewrapper. (3961) [Fabian Ruffy]
- Remove the inja submodule and replace it with a CMake fetchcontent instrumentation. (3959) [Fabian Ruffy]
- Remove boost variant from Tools. (3960) [Fabian Ruffy]
- Fix the following issues (3958) [Usha Gupta]

Version.txt

@@ -1 +1 @@
-1.2.3.8
+1.2.3.9