The MicroBuildBuildVSBootstrapper task only special-cases 'release'/'preview'
as public channels; any other name (including 'stable') is treated as an
internal channel and requires a vsdrop {prefix};{suffix} installer-manifest
URL. The public 'stable' channel manifest uses a download.visualstudio.microsoft.com
URL with no semicolon, so the task throws "drop url ... is invalid".

'stable' and 'release' resolve to the same public channel manifest
(VisualStudio.18.Release.chman), so this trains the same shipping bits while
fixing the bootstrapper build.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…#13992)

### Problem
PR #13923 changed `VisualStudio.ChannelName` from `int.main` to `stable`
to stop OptProf training against the moving `int.main` channel (which
caused assembly-version drift, e.g. System.Text.Json 10.0.0.4). Since
then the official build fails at **OptProf - Build VS bootstrapper**:

> The drop url
https://download.visualstudio.microsoft.com/.../VisualStudio.vsman is
invalid. Expected the drop url to have the format {prefix};{suffix}

### Root cause
The MicroBuild `MicroBuildBuildVSBootstrapper` task only special-cases
the literal channel names **`release`** and **`preview`** as public
channels. **Any other value — including `stable` — is assumed to be an
*internal* channel** whose installer-manifest URL is a vsdrop drop in
`{prefix};{suffix}` form. The public `stable` channel resolves
(`aka.ms/vs/18/stable/channel`) to `VisualStudio.18.Release.chman`,
whose installer-manifest URL is the public CDN
`download.visualstudio.microsoft.com/.../VisualStudio.vsman` (no
semicolon) -> the task throws.

The relevant task logic (`CreateConfigFile`):
https://dev.azure.com/devdiv/Engineering/_git/MicroBuild?path=/src/Tasks/BuildVSBootstrapper/plugin.ps1&line=286&lineEnd=307&lineStartColumn=1&lineEndColumn=1&lineStyle=plain&_a=contents

### Fix
Use `release` instead of `stable`. Both resolve to the **same** public
channel manifest (`VisualStudio.18.Release.chman`), so OptProf trains
against the identical shipping bits — but `release` is one of the two
special-cased names, so the task uses the aka.ms config template instead
of attempting to parse a vsdrop URL.

No behavioral change to trained bits; this only unblocks the
bootstrapper build.

Companion to #13993 (vs18.7).