XHTML mode is not honored

Description

Currently the plugin does not honor "XHTML mode": for example, if we have a configuration like this:

            new HtmlWebpackPlugin({
                filename: 'index.html',
                template: './src/index.html',
                xhtml: true,
            })

the plugin will still output html-style tags (e.g., <meta charset="utf-8"> instead of <meta charset="utf-8"/>).

What type of issue is this? (place an `x` in one of the `[ ]`)

[x] bug
[x] enhancement (feature request)
[ ] question
[ ] documentation related
[ ] testing related
[ ] discussion

Requirements (place an `x` in each of the `[ ]`)

[x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
[x] I've read and agree to the Code of Conduct.
[x] I've searched for any related issues and avoided creating a duplicate issue.

Bug Report

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Reproducible in:

slackhq/csp-html-webpack-plugin version: 4.0.0

node version: 12.17.0

OS version(s): any

Steps to reproduce:

See above - create a Webpack config with xhtml: true set for html-webpack-plugin

Expected result:

The generated HTML should have self-closing tags where appropriate, like

<meta charset="utf-8"/>

Actual result:

The generated markup does not honor xhtml: true:

<meta charset="utf-8">

slackhq / csp-html-webpack-plugin