fix(GlModal): Update modal position to centered

mentioned in commit f266b293

mentioned in merge request rpadovani/daintree!307 (merged)

mentioned in merge request gitlab!45424 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2122 (merged)

mentioned in merge request status-page!128 (merged)

mentioned in merge request gitlab-docs!1245 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2124 (merged)

mentioned in merge request status-page!129 (merged)

mentioned in merge request gitlab-docs!1270 (merged)