.gitlab: update default templates.

Adding some excerpt from our Code of Conduct to be "Considerate and
Respectful". Negative emotions are only making things worse and make
some contributors not even want to interact anymore with some reports.

Also in our MR template, add the mention that we don't want anything
AI-generated in or even anywhere near our project.

Both these topics are things which were recently discussed within the
team (mostly on IRC).

.gitlab/issue_templates/Default.md

@@ -6,6 +6,10 @@ It is important that you fill all the fields of the template.
 
 There are a few issues we get reports about quite frequently. If you want to check if what you have encountered is among these, please see: 
 https://gitlab.gnome.org/GNOME/gimp/-/issues/?sort=updated_desc&state=all&label_name%5B%5D=Duplication%20target&first_page_size=100
+
+**Code of Conduct**: "Be considerate and respectful" is our main rule.
+E.g. avoid negative emotional writing which only generates more upsetting
+interactions. Thanks!
 -->
 
 ### Environment/Versions

.gitlab/merge_request_templates/default.md

@@ -6,4 +6,7 @@
 - Make sure no trailing spaces or tabs are left out.
 
 - Check the following option when making your request:
-  "*Allow commits from members who can merge to the target branch.*" -->
+  "*Allow commits from members who can merge to the target branch.*"
+
+- No AI-generated contents allowed (neither code nor text, images…).
+  Only human created works please! -->