[IMP] theme_*: improve SEO by correcting heading tag hierarchy

* anelusia, artists, avantgarde, aviato, beauty, bewise, bistro,
bookstore, buzzy, clean, cobalt, enark, graphene, kea, kiddo, loftspace,
monglia, nano, notes, odoo_experts, orchid, paptic, real estate,
treehouse, vehicle, yes, zap

This commit adapts all the `xpath` expressions inside design themes to
target the new heading tags that were changed within the snippet files.

task-4349019

closes odoo/design-themes#1043

Related: odoo/odoo#191300
Related: odoo/enterprise#92978
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

theme_clean/views/snippets/s_carousel.xml

@@ -9,10 +9,10 @@
     <xpath expr="(//h2)[2]" position="replace" mode="inner">
         Simplify things
     </xpath>
-    <xpath expr="//h3" position="replace" mode="inner">
+    <xpath expr="(//h2)[3]" position="replace" mode="inner">
         Efficient team processes
     </xpath>
-    <xpath expr="//h6" position="replace"/>
+    <xpath expr="//div[hasclass('carousel-item')][3]//p" position="replace"/>
 
     <!-- Paragraphs -->
     <xpath expr="//p[hasclass('lead')]" position="replace" mode="inner">