Revert "hotfix(Windows Server Core LTSC2019) pin 2019 images to April version"

[lemeurherve]

Reverts #816

This PR removes the Windows Server Core version pinning so we can use its last image version published today as base instead of the one from April 2024.

From microsoft/Windows-Containers#493 (comment):

The updated images released as part of the July 2024 security update today include the fix for this issue.

Confirmed by building this from another host than Windows:

# OK with docker buildx build --platform windows/amd64 -t test .
FROM --platform=windows/amd64 mcr.microsoft.com/windows/servercore:ltsc2019 AS latest
COPY README.md README.md

# OK with docker buildx build --platform windows/amd64 -t test .
FROM --platform=windows/amd64 mcr.microsoft.com/windows/servercore:ltsc2019@sha256:41f42aa4ad39d85e4d30642b8111ca6454ca2275f188f012934b9afbaf63a647 AS latestdigest
COPY README.md README.md

Ref:

• Unable to build an extension of inbound-agent:*ltsc2019 #815 (comment)