Update dependency `handlebars` to v6 for `mdman`

[weihanglo]

Problem

Renovate bot found a newer version available for dependency handlebars in #14763. handlebars is used by mdman for templatizing and generating man pages.

However, the new version has some API changes that need to be fixed:

Build failure. Click to See details

   Compiling mdman v0.0.0 (/Users/runner/work/cargo/cargo/crates/mdman)
error[E0639]: cannot create non-exhaustive struct using struct expression
  --> crates/mdman/src/hbs.rs:27:19
   |
27 |       let options = DirectorySourceOptions {
   |  ___________________^
28 | |         tpl_extension: ".md".to_string(),
29 | |         hidden: false,
30 | |         temporary: false,
31 | |     };
   | |_____^

Steps

1. Clone rust-lang/cargo
2. Run cargo build-man
3. Fix the build failure.

Possible Solution(s)

Look into what have been changed in handlebars and update accordingly.

Version

7421ccf9159d5ac43883c53abe41ef1da657acfd

[hcbarker]

I can do this!