Pin StefanScherer/windows_2019_docker box to 2020.04.15

What does this MR do?

Pins the Windows vagrant base box to 2020.04.15.

Why was this MR needed?

Later versions of the StefanScherer/windows_2019_docker box have upgraded Docker to a version (more recent than 19.03.05) that doesn't deal well with the C:\GitLab-Runner shared volume symlink.

.\ci\build_release_windows_images.ps1 was failing with the following error:

unable to prepare context: unable to evaluate symlinks in context path: CreateFile \\vboxsvr\C:_GitLab-Runnerdockerfiles: The network name cannot be found.

What's the best way to test this MR?

1. Recreate the vagrant image with:

   vagrant destroy -f; vagrant up

2. SSH into the vagrant VM:

   vagrant ssh

3. Enter a new powershell session.

4. Build the Windows Docker image:

   cd $Env:RUNNER_SRC
   $Env:CI_REGISTRY_IMAGE="localhost:5000/gitlab"
   $Env:WINDOWS_VERSION = 'servercore1809'
   $Env:GIT_LFS_256_CHECKSUM= "5c12db9728b53cba23e5f58f4a53d88cb2132e82fa1de0f8a79ce9d112e4d396"
   $Env:GIT_256_CHECKSUM= "bd91db55bd95eaa80687df28877e2df8c8858a0266e9c67331cfddba2735f25c"
   $Env:GIT_VERSION= "2.21.0"
   $Env:GIT_VERSION_BUILD= "1"
   $Env:GIT_LFS_VERSION= "2.7.1"
   $Env:PWSH_VERSION= "7.0.1"
   $Env:PWSH_256_CHECKSUM= "e652a8e0f7d088106ea018d4b9e02373d4331907efa64a60dc32e097b165d8fd"
   $Env:SKIP_CLEANUP=1
   .\ci\build_release_windows_images.ps1

The image should finish building successfully.

What are the relevant issue numbers?

Closes #27115 (closed)