fix some frontmatter errors (lastmod; remove .Site.LastChange)

In 6932cbd I forgot to also remove `.Site.Lastchange` from
`layouts/sitemap.xml`

which resulted in:

~~~
ERROR deprecated: .Site.LastChange was deprecated in Hugo v0.123.0 and
will be removed in Hugo 0.136.0. Use .Site.Lastmod instead.
~~~

content/equipment/radio-stuff/antennas/chameleon-emcomm-3-portable/index.md

@@ -1,8 +1,8 @@
 ---
 title: Chameleon EMCOMM 3 Portable
-summary: 
+summary:
 date: 2023-04-23T08:35:46+02:00
-lastmod: 
+# lastmod:
 #categories:
 #- amateur-radio
 #tags:
@@ -19,8 +19,6 @@ showWordCount: false
 # site_exclude: true
 
 draft: true
-
 ---
 
 A very rugged and good working antenna, a bit heavy for my taste.
-

content/equipment/radio-stuff/antennas/homebrew-80m-hw-endfed/index.md

@@ -1,8 +1,8 @@
 ---
 title: Homebrew 80m half-wave end-fed antenna
-summary: 
+summary:
 date: 2023-05-01T11:44:57+02:00
-lastmod: 
+# lastmod:
 #categories:
 #- amateur-radio
 #tags:
@@ -21,7 +21,6 @@ showWordCount: false
 # site_exclude: true
 
 draft: true
-
 ---
 
 This is the long version of my portable homebrew half-wave end-fed antenna.

layouts/sitemap.xml

@@ -4,7 +4,7 @@
     {{ if not .Params.site_exclude }}
       <url>
         <loc>{{ .Permalink | absURL }}</loc>{{ if not .Lastmod.IsZero }}
-        <lastmod>{{ safeHTML ( .Lastmod.Format "2006-01-02T15:04:05-07:00" ) }}</lastmod>{{ else }}<lastmod>{{ .Site.LastChange.Format "2006-01-02T15:04:05-07:00" }}</lastmod>{{ end }}{{ with .Sitemap.ChangeFreq }}
+        <lastmod>{{ safeHTML ( .Lastmod.Format "2006-01-02T15:04:05-07:00" ) }}</lastmod>{{ else }}<lastmod>{{ .Site.Lastmod.Format "2006-01-02T15:04:05-07:00" }}</lastmod>{{ end }}{{ with .Sitemap.ChangeFreq }}
         <changefreq>{{ . }}</changefreq>{{ else }}<changefreq>weekly</changefreq>{{ end }}{{ if ge .Sitemap.Priority 0.0 }}
         <priority>{{ .Sitemap.Priority }}</priority>{{ else }}<priority>0.5</priority>{{ end }}
       </url>