CHEF-25807 - Remove SLA text from README.md

[Saburesh07]

This pull request removes the oft-misleading Chef SLA text from the README.md file.

This action is being taken as part of the 2025 Repo Standardization Initiative.

As Progress Chef makes a best effort to respond to issues and pull requests in a timely manner, and prioritizes bugfixes and security updates on a customer-driven basis (which may span repos, or have no repo footprint at all), we no longer support the concept of a Service Level Agreement (SLA) on a repository-centric basis. For further details, see Repo SLA Removal FAQ.

[chef-expeditor]

Hello Saburesh07! Thanks for the pull request!

Here is what will happen next:

1. Your PR will be reviewed by the maintainers.
2. Possible Outcomes
   a. If everything looks good, one of them will approve it, and your PR will be merged.
   b. The maintainer may request follow-on work (e.g. code fix, linting, etc). We would encourage you to address this work in 2-3 business days to keep the conversation going and to get your contribution in sooner.
   c. Cases exist where a PR is neither aligned to Chef InSpec's product roadmap, or something the team can own or maintain long-term. In these cases, the maintainer will provide justification and close out the PR.

Thank you for contributing!