Bump main to 18.7, add vs18.6 to merge flow

[MichalPavlik]

Release 18.6 — Phase 1.3 + Phase 2

Tracking issue: #13468

Changes

1. **\�ng/Versions.props**: Bump \VersionPrefix\ from \18.6.0\ → \18.7.0\
2. **.config/git-merge-flow-config.jsonc**: Insert \�s18.6\ into the merge chain (\�s18.5 → vs18.6 → main)

Not changed (no-op)

• \�s-insertion.yml: Auto-targets VS \main\ for all \�s*\ branches — no branch-specific mapping needed
• \�s-insertion-experimental.yml: Already has \main/
  el/insiders/
  el/stable\ target options

[Copilot]

Pull request overview

Updates MSBuild’s main branch versioning and merge automation to reflect the new VS 18.6 release branch and the next development version (18.7).

Changes:

• Bump eng/Versions.props VersionPrefix from 18.6.0 to 18.7.0.
• Extend the merge-flow chain so vs18.5 merges into vs18.6, and vs18.6 merges into main.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
eng/Versions.props	Updates the main-branch version prefix to 18.7.0.
.config/git-merge-flow-config.jsonc	Inserts vs18.6 into the automated branch merge chain before main.

[rainersigwald]

This is blocked on https://dev.azure.com/dnceng/internal/_git/maestro-configuration/pullrequest/59808