Fix OpenGraph modified_time to use .Params.lastmod

[versha890]

This PR fixes the issue with the OpenGraph article:modified_time meta tag.
Previously, regenerating the site updated the modification time incorrectly for all posts.
Now, it uses .Params.lastmod to reflect the actual last modified date set in the post front matter.

##Changes

Updated _partials/opengraph.html:
{{ with .Params.lastmod }}
<meta property="article:modified_time" content="{{ . | time.Format "2006-01-02T15:04:05Z07:00" }}" />
{{ end }}

[choo121600]

It seems like there are quite a few unnecessary changes.

I’ve noticed similar patterns across several of your PRs.
A bit of additional review yourself before submitting could help improve the overall quality.

[versha890]

Thank you for the feedback.
I understand your point and I’ll be more careful with self-review before submitting future PRs.
I’ll also recheck this PR and reduce any unnecessary changes where possible.