livecheck: add content parameter to strategies

This renames `provided_content` parameters in livecheck strategy
`find_versions` methods to `content` (for the sake of brevity) and
adds the parameter to those without it. This will be used to implement
a caching solution in the future, to avoid fetching duplicate URLs
more than once in a given run. In the interim time, this allows us to
expand tests for `find_versions` methods, which brings some of these
strategies up to 100% code coverage.

This change also allows us to remove `allow_incompatible: true` from
related `find_versions` method signatures, as they align with what's
outlined in `Strategic`. The only exception is
`ExtractPlist#find_versions`, which will always differ due to having a
`cask` parameter and the `url` parameter being optional (other
strategies require a URL to function).

Other notable changes:

* `GithubReleases.find_versions` calls `GitHub::API.open_rest` and that
parses the JSON response by default, so we have to use a
`parse_json: false` argument to ensure that `content` is a string and
handle the JSON parsing in the strategy.
* Similarly, `GithubLatest` uses `GitHub.get_latest_release`, which
doesn't support options like `parse_json: false`. `get_latest_release`
generates the API URL from the GitHub username and repository before
calling `GitHub::API.open_rest` and we already generate the same URL
in the strategy, so this switches to an `open_rest` call instead.
* `Launchpad.find_versions` uses `DEFAULT_REGEX` as the `regex`
parameter's default value but this only applies if the `regex`
argument is omitted. If `regex: nil` is used in the `find_versions`
call, then `regex` is `nil` instead of the default regex, which isn't
what we want. This sets the `regex` parameter's default value to `nil`
and uses `regex || DEFAULT_REGEX` in the `PageMatch.find_versions`
arguments instead, aligning with other strategies.
* The `Xorg` strategy has its own built-in caching (which will be
replaced by the general caching solution in the future), so it uses
`content || cached_content` for now.