ci: bump git-for-windows/setup-git-for-windows-sdk from v1 to v2

ci: bump git-for-windows/setup-git-for-windows-sdk from v1 to v2

The v1 of `git-for-windows/setup-git-for-windows-sdk` runs on
Node.js 20, which GitHub is phasing out of the Actions runners.
v2 moves the action to Node.js 24 so that the CI jobs relying on
a Git for Windows SDK keep working once Node.js 20 is removed.

The risk is very low: v2 contains no functional changes to the
SDK setup itself, only the runtime upgrade. The action still
provisions the same minimal SDK and exposes the same outputs.
The sole precondition is a recent Actions Runner (>= 2.327.1),
which the github.com-hosted runners already satisfy.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>