Introduce build target for Git's previous version

To begin deploying Gitaly’s Git version from master, we introduced the GIT_VERSION_MASTER and GIT_VERSION_PREV variables. Previously, we didn't add a build target for GIT_VERSION_PREV because we did not yet have a stable release other than 2.50. Now that both GIT_VERSION_MASTER and GIT_VERSION_PREV point to stable versions, we can safely remove 2.50 as the fallback option.