fix: revert upgrade to gitlab-code-parser v0.20.0

What does this MR do and why?

The gitlab-code-parser v0.20.0 upgrade (chore(deps): update module gitlab.com/gitlab-or... (!771 - merged)) is causing an error.

Note: I discovered the error on my local machine. There are no errors in production yet, but it seems the MR introducing the upgrade (!771 (merged)) hasn't been released.

Error trace

Full error log: gitlab_elastic_indexer_error.txt

Key logs:

thread '<unnamed>' panicked at crates/chunker/src/split_code_chunker.rs:139:42:
index out of bounds: the len is 1 but the index is 1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

thread '<unnamed>' panicked at library/core/src/panicking.rs:225:5:
panic in a function that cannot unwind
stack backtrace:
   0:        0x102c193b0 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hf35f9734f9a29483
   1:        0x102c0e454 - core::fmt::write::h60ec6633daab7b35
   2:        0x102c18c68 - std::io::Write::write_fmt::hc29709fdab2e34e2
   3:        0x102c191c8 - std::sys::backtrace::BacktraceLock::print::hca95bffd78053951
   4:        0x102c18614 - std::panicking::rust_panic_with_hook::h557a23724a5de839
   5:        0x102c379cc - std::panicking::begin_panic_handler::{{closure}}::h269cace6208fef05
   6:        0x102c37964 - std::sys::backtrace::__rust_end_short_backtrace::h5be0da278f3aaec7
   7:        0x102c38050 - __rustc[de2ca18b4c54d5b8]::rust_begin_unwind
   8:        0x102de75d4 - core::panicking::panic_nounwind_fmt::h9825e2aa83719df7
   9:        0x102de7630 - core::panicking::panic_nounwind::h4cc28a4411926d9d
  10:        0x102de75e8 - core::panicking::panic_cannot_unwind::ha4e3ecab6cb0371c
  11:        0x102c09900 - _chunker_chunk_files
  12:        0x102c02a50 - __cgo_5cdf7be2fef5_Cfunc_chunker_chunk_files
thread caused non-unwinding panic. aborting.
SIGABRT: abort
PC=0x18ded25b0 m=5 sigcode=0
signal arrived during cgo execution

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.