Migrate from .tool-versions to mise.toml

What does this MR do?

Migrates from .tool-versions to mise.toml as the config file for specifying our local dev tool dependencies.

Also updates the Renovate config so its regex manager reads tool versions from mise.toml instead of .tool-versions.

Why was this MR needed?

.tool-versions is the format used by asdf, a tool we no longer use. It's supported by mise but isn't its native format. Moving to mise.toml aligns us properly with mise as our local dev environment tool going forward.

What's the best way to test this MR?

Run mise install locally and confirm the tools resolve (e.g. mise ls).

What are the relevant issue numbers?

N/A