experienced this issue when attempting to update to 4.5.0 from 4.2.5

fix: add missing SQL to update

`ALTER TABLE mdl_communication DROP INDEX mdl_comm_con_ix;`

steps taken:

1. updated bitnami moodle image from 4.2.5 to 4.5.0
   no change
   tried downloading the tgz file from download.moodle.org but got a html file instead.
2. clicked on the download button in the web browser, cancelled the download, copied the download link
3. downloaded the tgz file from download.moodle.org to server
4. extracted files from tgz file
5. turned on maintenance mode
6. swapped some directories around and copied config.php file
7. moodle is now allowing for the update process to start
8. moodle complains about missing plugins instead of downloading them.
9. swap more directories around for missing files.
10. click update.

get the following error message:
column "communication->contextid" cannot be modified. Dependency found with index "mdl_comm_con_ix (contextid)"

More information about this error
Debug info:
Error code: ddldependencyerror×Dismiss this notification
Stack trace: * line 734 of /lib/ddl/database_manager.php: ddl_dependency_exception thrown

• line 591 of /lib/ddl/database_manager.php: call to database_manager->check_field_dependencies()
• line 634 of /lib/ddl/database_manager.php: call to database_manager->change_field_type()
• line 830 of /lib/db/upgrade.php: call to database_manager->change_field_notnull()
• line 1871 of /lib/upgradelib.php: call to xmldb_main_upgrade()
• line 525 of /admin/index.php: call to upgrade_core()