fix: Add /v2 suffix to module path.

The module directive in go.mod and all internal imports were missing the required /v2 suffix, making the module unusable with Go tooling.

This change is largely generated with:

sed -r -i '' -e 's#"gitlab.com/gitlab-org/api/client-go#"gitlab.com/gitlab-org/api/client-go/v2#' \
    $(fgrep -rIl '"gitlab.com/gitlab-org/api/client-go')

I used the following grep command to check for unmigrated occurrences:

grep -rIE '(^|[^/])gitlab.com/gitlab-org/api/client-go($|[^/]|/[^v]|/v[^2])' --exclude-dir .git

• (^|[^/]) approximates a negative lookbehind: only match occurrences not preceded by /, which excludes https:// URLs
• ($|[^/]|/[^v]|/v[^2]) approximates a negative lookahead: only match occurrences not followed by /v2

Fixes: #2239 (closed)