chore: Create GH action to auto bump rustc

[devin-ai-integration]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

Ported from rust-eventsource-client (sha f26dc37a6eb60eb31f3e106313d64b21de2c1949).

Describe the solution you've provided

This PR adds an automated workflow to keep the Minimum Supported Rust Version (MSRV) up to date, ported from rust-eventsource-client.

Changes include:

1. New check-rust-versions.yml workflow - Runs daily and on-demand to check if the MSRV needs updating. It fetches Rust release data from endoflife.date and targets the antepenultimate (3rd most recent) stable release. When an update is needed, it automatically creates a PR updating both the env file and Cargo.toml.

2. New .github/variables/rust-versions.env - Centralizes the MSRV (currently 1.88) so it can be referenced dynamically.

3. Updated ci.yml - Replaced hardcoded 1.88 with dynamic version lookup from the env file in both ci-build and musl-build jobs.

4. Updated README MSRV policy - Changed from "six months old" to "latest stable release plus two prior minor releases" to match the new automated policy.

Describe alternatives you've considered

Could continue manually updating MSRV, but automation reduces maintenance burden and ensures consistency.

Additional context

Requested by @keelerm84. Devin session

Human review checklist:

• Verify paths in check-rust-versions.yml correctly reference launchdarkly-server-sdk/Cargo.toml for this workspace structure
• Confirm the MSRV policy change in README aligns with team expectations

[devin-ai-integration]

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