Trim the migration guide to genuine v1-to-v2 breaking changes

The guide had accumulated entries that are not v1-to-v2 migration
items: new v2 feature notes, "what did not change" reassurance
sections, spec-adoption commentary, deprecations of APIs that still
work, and internal fixes with nothing for a migrator to change. Remove
those, including the whole Deprecations group; the two deprecation
consequences a migrator does act on (warnings-as-errors test runs, and
client calls that die on the default 2026-era connection) fold into
the testing section.

Correct code that was wrong for v1.x or stale for current main --
including the `mcp.types` alias now being permanent -- and verify every
remaining before/after example against the v1.x and main trees. Reframe
the 2026-era notes as behavior changes migrated code sees on v2's
default connection, fold closely related entries together, standardize
on the documented `from mcp.server import MCPServer` and `mcp.types`
imports, regenerate the index and suggested-order blocks, and cut
history and over-explanation throughout.

Also drop two whats-new.md links whose target sections this removes
(the strict docs build aborts on missing anchors).

No-Verification-Needed: documentation-only change

Author: maxisbey