Add experimental config to disable TLS1.3

[stevejgordon]

Workaround for #5951

There is currently an issue for users on Windows 11, targetting .NET 5.0 where TLS negotiation to some cloud regions is failing when TLS 1.3 is enabled. A workaround is to disable TLS 1.3 at the OS level but that is problematic as it affects all client-side apps.

This PR adds a temporary and experimental configuration option that may be used to disable TLS 1.3 in the client directly. This setting affects only .NET Core and .NET 5.0+ targets. It is intended to be used purely as a short term workaround until changes can be made at the cloud proxy layer to resolve the negotiation issue.

Example usage:

var credentials = new BasicAuthenticationCredentials("elastic", "password"); 
var pool = new CloudConnectionPool("CloudId", credentials); 
var esClient = new ElasticClient(new ConnectionSettings(pool).UnsafeDisableTls13());

[github-actions]

The backport to 7.15 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-7.15 7.15
# Navigate to the new working tree
cd .worktrees/backport-7.15
# Create a new branch
git switch --create backport-6025-to-7.15
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 9dbd7869447ae88de4a0796001b81dfe672429eb
# Push it to GitHub
git push --set-upstream origin backport-6025-to-7.15
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.15

Then, create a pull request where the base branch is 7.15 and the compare/head branch is backport-6025-to-7.15.