Raise the minimum git requirement to 2.25

As I was working on !2113 (merged), I realized 2 things:

1. Gitaly still have the minimum Git requirement of 2.22.0 inside the CI matrix

2. The CI matrix has the wrong naming convention

test:go1.14-git-2.26-ruby-2.6:
  image: registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6-golang-1.14-git-2.26
  <<: *test_definition

test:go1.14-git-2.24-ruby-2.6:
  image: registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6-golang-1.14-git-2.24
  <<: *test_definition

test:go1.14-git-2.22-ruby-2.6:
  image: registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6-golang-1.14-git-2.26 << should be 2.22
  <<: *test_definition

test:go1.13-git-2.26-ruby-2.6:
  image: registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6-golang-1.13-git-2.26
  <<: *test_definition

test:go1.13-git-2.24-ruby-2.6:
  image: registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6-golang-1.13-git-2.22 << should be 2.24
  <<: *test_definition

test:go1.13-git-2.22-ruby-2.6:
  image: registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6-golang-1.13-git-2.24 << should be 2.22
  <<: *test_definition

So here is a proposal:

1. As 2.27.0 is at the horizon, let's raise the minimum git version from 2.22 to 2.25 This should enable Gitaly to start accepting some key configuration at its default value (such as core.commitGraph=true).

2. Fix the build matrix naming. !2177 (closed)

3. Consider a more aggressive minimum git version. Since Gitaly is shipped with it's own patched git version anyway, its better to stay up-to-date. Some of the features in 2.27 (i.e. bloomfilter inside commit-graph) could be quite good to have but if you need to stay below 2.26, these wont be available.