Remove wkhtmltopdf as a dependency from the README

Track the full paths of attachments in the Attachment object (#71)

<!-- Please add a title in the form of a great git commit message in the
imperative mood (https://cbea.ms/git-commit/) -->

**What is changing**:
1. Disambiguate `Filename` and `Filepath` in the `Attachment` object.
2. Replace relative export paths with absolute ones.

**Why this change is being made**:
1. Previously, we were computing the absolute path of the attachments in
multiple places in different ways depending on the options given and
whether the attachment was converted or not. This was confusing and
error prone (see #70).
2. As a result of the first change, we are using the copied path as the
file to embed or reference. When embedding in PDFs, relative paths do
not work, so we need to specify the copied path (under the export path
in the case of the `--copy-attachments` option) as an absolute path.

**Related issue(s)**: Fixes #70 

**Follow-up changes needed**: None AFAIK

**Is the change completely covered by unit tests? If not, why not?**:
No, similarly to converting the attachments path to an absolute path, we
are not testing failures of the `filepath.Abs` function.

Include the invocation in the results string

Update DATA-DOG/go-sqlmock

Upgrade tagatac/goheif

Bump the minimum supported Go version to 1.20 (#65)

**What is changing**:
- Bump the Go version to 1.20
- Use that specific version in the GH action tests

**Why this change is being made**: jessevdk/go-flags requires Go 1.20:
https://github.com/jessevdk/go-flags/blob/c02e333e441eb1187c25e6d689d769d499ec2a0b/go.mod#L3

**Related issue(s)**: None

**Follow-up changes needed**: None

**Is the change completely covered by unit tests? If not, why not?**:
N/A

Update dependencies

Update dependencies

Fix misspelling in exectest docstring

Add v2 to the module name

Following best practices for releasing modules v2 or higher:
https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher