Replace .Text with .PlainText

[Arty2]

Replace .Text with .PlainText in alt to conform with the CommonMark specification: “Only the plain string content is rendered, without formatting.”

[CLAassistant]

All committers have signed the CLA.

[jmooring]

You have a failing test that you can correct with this change:

diff --git a/hugolib/content_render_hooks_test.go b/hugolib/content_render_hooks_test.go
index 7f2c35032..a06dda3b2 100644
--- a/hugolib/content_render_hooks_test.go
+++ b/hugolib/content_render_hooks_test.go
@@ -279,7 +279,7 @@ Image: ![alt-"<>&](/destination-"<> 'title-"<>&')
                        if enabled {
                                b.AssertFileContent("public/index.html",
                                        "Link: <a href=\"/destination-%22%3C%3E\" title=\"title-&#34;&lt;&gt;&amp;\">text-&quot;&lt;&gt;&amp;</a>",
-                                       "img src=\"/destination-%22%3C%3E\" alt=\"alt-&quot;&lt;&gt;&amp;\" title=\"title-&#34;&lt;&gt;&amp;\">",
+                                       "img src=\"/destination-%22%3C%3E\" alt=\"alt-&#34;&lt;&gt;&amp;\" title=\"title-&#34;&lt;&gt;&amp;\">",
                                        "&gt;&lt;script&gt;",
                                )
                        } else {

Then test locally with go test ./....

Finally, change the commit message to adhere to these guidelines.
https://github.com/gohugoio/hugo/blob/master/CONTRIBUTING.md#git-commit-message-guidelines.

Something like:

tpl/tplimpl: Use PlainText for alt tag in image render hook