appveyor: bump VS2008 jobs to VS2010

appveyor: bump VS2008 jobs to VS2010

VS2008 has been partly broken for a while with its shared-debug builds
crashing on startup. Its compiler output (UTF-16 HTML) was also barely
readable even after conversion. It's also the only platform in CI
missing `stdint.h`.

This patch migrates a VS2008 job to VS2010 and drops another that
already had a VS2010 equivalent.

We recommend switching to VS2010 or newer when using MSVC to build curl.

Ref: #15907
Closes #15934