Update release dates on the release notes

In the future, perhaps it would be better to just note the planned
release date when the drafting the new release notes for the next
minors.

general/releases/3.11/3.11.8.md

@@ -9,7 +9,7 @@ moodleVersion: 3.11.8
 ---
 [Releases](/general/releases) > Moodle 3.11.8 release notes
 
-Release date: Not yet released
+Release date: 11 July 2022
 
 Here is [the full list of fixed issues in 3.11.8](https://tracker.moodle.org/secure/IssueNavigator!executeAdvanced.jspa?jqlQuery=project+%3D+mdl+AND+resolution+%3D+fixed+AND+fixVersion+in+%28%223.11.8%22%29+ORDER+BY+priority+DESC&runQuery=true&clear=true).
 

general/releases/3.9/3.9.15.md

@@ -9,7 +9,7 @@ moodleVersion: 3.9.15
 ---
 [Releases](/general/releases) > Moodle 3.9.15 release notes
 
-Release date: Not yet released
+Release date: 11 July 2022
 
 Here is [the full list of fixed issues in 3.9.15](https://tracker.moodle.org/secure/IssueNavigator!executeAdvanced.jspa?jqlQuery=project+%3D+mdl+AND+resolution+%3D+fixed+AND+fixVersion+in+%28%223.9.15%22%29+ORDER+BY+priority+DESC&runQuery=true&clear=true).
 

general/releases/4.0/4.0.2.md

@@ -8,7 +8,7 @@ sidebar_position: 2
 moodleVersion: 4.0.2
 ---
 
-Release date: Not yet released
+Release date: 11 July 2022
 
 Here is [the full list of fixed issues in 4.0.2](https://tracker.moodle.org/secure/IssueNavigator!executeAdvanced.jspa?jqlQuery=project+%3D+mdl+AND+resolution+%3D+fixed+AND+fixVersion+in+%28%224.0.2%22%29+ORDER+BY+priority+DESC&runQuery=true&clear=true).