MDL-69479 upgrade: Change all 4.0 occurrences to 3.10

Under parallel development, when something is worth being commented
in upgrade.txt notes, it will be applied to all the development branches
using the "earliest" one to be released. So, if something changes
in 3.10 (and also is applied to master), the upgrade.txt notes will
show 3.10 in both branches.

Of course, if something only goes to one dev branch (say master), then the
master version will be used in the notes (4.0 in this case).

admin/tool/log/upgrade.txt

@@ -11,5 +11,5 @@ information provided here is intended especially for developers.
 
 === 3.6 ===
 
-* The legacy log store is in its first stage of deprecation and is due for removal in Moodle 4.0. Please use one of
+* The legacy log store is in its first stage of deprecation and is due for removal in Moodle 3.10. Please use one of
   the other log stores such as "standard" and "database".

cache/upgrade.txt

@@ -1,7 +1,7 @@
 This files describes API changes in /cache/stores/* - cache store plugins.
 Information provided here is intended especially for developers.
 
-=== 4.0 ===
+=== 3.10 ===
 * The function supports_recursion() from the lock_factory interface has been deprecated including the related implementations.
 * The function extend_lock() from the lock_factory interface has been deprecated without replacement including the related
   implementations.

calendar/upgrade.txt

@@ -1,7 +1,7 @@
 This files describes API changes in /calendar/* ,
 information provided here is intended especially for developers.
 
-=== 4.0 ===
+=== 3.10 ===
 * The core_calendar\local\event\value_objects\times_interface class now has new method get_usermidnight_time() which
   returns the user midnight time for a given event.
 

course/upgrade.txt

@@ -1,7 +1,7 @@
 This files describes API changes in /course/*,
 information provided here is intended especially for developers.
 
-=== 4.0 ===
+=== 3.10 ===
 
 * The function make_categories_options() has now been deprecated. Please use \core_course_category::make_categories_list() instead.
 

grade/grading/form/upgrade.txt

@@ -1,7 +1,7 @@
 This files describes API changes in /grade/grading/form/* - Advanced grading methods
 information provided here is intended especially for developers.
 
-=== 4.0 ===
+=== 3.10 ===
 
 * Removed gradingform_provider.
 * Removed the following deprecated functions:

h5p/upgrade.txt

@@ -1,7 +1,7 @@
 This files describes API changes in core libraries and APIs,
 information provided here is intended especially for developers.
 
-=== 4.0 ===
+=== 3.10 ===
 * Added a new cache for h5p_library_files (MDL-69207)
 
 === 3.9 ===

lib/upgrade.txt

@@ -1,7 +1,7 @@
 This files describes API changes in core libraries and APIs,
 information provided here is intended especially for developers.
 
-=== 4.0 ===
+=== 3.10 ===
 * Added function setScrollable in core/modal. This function can be used to set the modal's body to be scrollable or not
   when the modal's height exceeds the browser's height. This is also supported in core/modal_factory through the
   'scrollable' config parameter which can be set to either true or false. If not explicitly defined, the default value

question/behaviour/upgrade.txt

@@ -1,6 +1,6 @@
 This files describes API changes for question behaviour plugins.
 
-=== 4.0 ===
+=== 3.10 ===
 
 1) The slot parameter of method M.core_question_engine.init_submit_button now removed.
    The method will get the unique id by using the 'Check' button element.

search/upgrade.txt

@@ -1,7 +1,7 @@
 This files describes API changes in /search/*,
 information provided here is intended especially for developers.
 
-=== 4.0 ===
+=== 3.10 ===
 
 * Search indexing now supports sending multiple documents to the server in a batch. This is implemented
   for the Solr search engine, where it significantly increases performance. For this to work, engines