Refresh ROADMAP.md for the shipped 2026-07-28 support

[koic]

Motivation and Context

The roadmap predates the 2026-07-28 implementation work and has drifted from the released SDK. The 2026-08-15 SEP-1730 tier audit flagged the drift while evaluating the roadmap requirement:

• "Current Status" claims only 2025-06-18 and 2025-11-25 support, but the SDK now completes the 2026-07-28 revision and passes all scored server and client conformance scenarios for each revision's requirement set.
• The SEP-2575 stateless lifecycle rewrite was described as reserved for 2.0, but it shipped during 1.x with both lifecycle eras served side by side; the few incompatible adjustments shipped in a minor release under the spec-conformance and security exceptions described in VERSIONING.md, as recorded in the 1.2.0 CHANGELOG entry.
• Deprecation warnings for Roots/Sampling/Logging (SEP-2577) were described as future work but have shipped (Deprecate roots, sampling, and logging per SEP-2577 #406, Stop negotiating modern protocol versions through the initialize handshake #516).
• The documentation section undersold the coverage: all implemented non-experimental features are documented with examples (Document non-text content types, binary resources, and elicitation schema options #472, [Doc] Add prompts/resources list-changed notification examples to README.md #515).

The refreshed text records the implemented spec revisions and how the 2026-07-28 revision actually shipped, and keeps the tasks extension (SEP-2663, #391) and the removal of legacy-era support as the remaining tracked items. Gem versions are deliberately not named: the roadmap describes the current state of the SDK, and the version-to-revision mapping already lives in CHANGELOG.md.

How Has This Been Tested?

Documentation-only change; no code paths are affected. Statements were cross-checked against the v1.2.0 CHANGELOG entry and the 2026-08-15 tier audit results (server 67/67 and client 50/50 scored conformance across the 2025-11-25 and 2026-07-28 requirement sets).

Breaking Changes

None. This only updates ROADMAP.md.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed