Update htslib and add libcurl variant (#14504)

* Update htslib and add libcurl variant

This PR updates htslib to version 1.10.2 and adds a libcurl variant. The
libcurl variant defaults to True because, while it is optional, it is
highly recommended by the project developers.

Other things done:

- be consistent with quotes

* Change version in variant description

Apparently, `spack info` does not like the `@` character in a variant
description.

```
==> Error: Incomplete color format: '@' in
@1.3:.
```