ci: use release-plz fork with semver_check_features

[DaleSeo]

Motivation and Context

This PR adds a release-plz.toml with semver_check_features = ["default"] so release-plz only checks the default feature set for semver compatibility, and updates the workflow to install release-plz from a fork that supports this config option. Once upstream ships release-plz/release-plz#2757, we can switch back to the official action.

How Has This Been Tested?

Install release-plz from the fork that implements semver_check_features:

cargo +stable install --git https://github.com/DaleSeo/release-plz --branch feat/semver-check-features release-plz

Then run release-plz update -p rmcp in the rust-sdk repo:

❯ release-plz update
2026-03-25T20:55:01.903559Z INFO using release-plz config file release-plz.toml
2026-03-25T20:55:01.931376Z WARN no upstream configured for branch ci/release-plz-fork
2026-03-25T20:55:01.943701Z INFO downloading packages from cargo registry crates.io
Updating crates.io index
2026-03-25T20:55:03.111154Z WARN no upstream configured for branch ci/release-plz-fork
2026-03-25T20:55:03.156279Z INFO determining next version for rmcp-macros 1.2.0
2026-03-25T20:55:03.264296Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp-macros
2026-03-25T20:55:03.412205Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp-macros
2026-03-25T20:55:03.590865Z INFO determining next version for rmcp 1.2.0
2026-03-25T20:55:03.699220Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp
2026-03-25T20:55:03.834641Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp
2026-03-25T20:55:03.969380Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp
2026-03-25T20:55:04.102573Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp
2026-03-25T20:55:04.235126Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp
2026-03-25T20:55:04.365835Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp
2026-03-25T20:55:04.506730Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp
2026-03-25T20:55:04.648640Z INFO Getting packaged files for crate at /var/folders/v2/5j2zr9qs7c76ph0yxn53d_9r0000gp/T/.tmpWHZ4uy/rust-sdk/crates/rmcp
2026-03-25T20:55:04.845396Z INFO Checking API compatibility with cargo-semver-checks...
2026-03-25T20:55:19.511118Z INFO rmcp-macros: next version is 1.3.0
2026-03-25T20:55:19.513573Z INFO rmcp: next version is 1.3.0 (✓ API compatible changes)
2026-03-25T20:55:19.891889Z WARN no upstream configured for branch ci/release-plz-fork
* `rmcp-macros`: 1.2.0 -> 1.3.0
* `rmcp`: 1.2.0 -> 1.3.0 (✓ API compatible changes)

Breaking Changes

None

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

Additional context