ci: Bump CI_GIT_VERSION to 2.51

CI_GIT_VERSION controls the tag of the builder image we pull to execute CI jobs. The builder images for Gitaly are constructed from the matrix of toolchain versions defined here.

We recently modified the matrix to select Git 2.51 as the Git version. Since the Gitaly CI configuration was stuck on Git 2.49, we weren't able to use any of the new images.

Bump CI_GIT_VERSION to align with what we're building in the matrix.