Added mention about LTS updates in the update doc

[mnocon]

Target: master, 4.6

It's been reported to me that not all Partners are aware that LTS Updates need to be updated separately - so I'm adding a mention about it at the end of the update doc.

Things done in addition:

1. Added mention about notifying Support
2. Renamed Support portal to Service portal

Both points consulted with @mateuszbieniek

[github-actions[bot]]

Preview of modified Markdown:

• docs/getting_started/install_ibexa_dxp.md
• docs/infrastructure_and_maintenance/security/reporting_issues.md
• docs/infrastructure_and_maintenance/security/security_checklist.md
• docs/resources/contributing/report_and_follow_issues.md
• docs/resources/release_process_and_roadmap.md
• docs/snippets/update/notify_support.md
• docs/update_and_migration/from_4.6/update_from_4.6.md

[mnocon]

@dabrt than you for your comments, I've applied all of them @adriendupuis thank you for comment as well 😄 I've unified all the occurances to "Service portal" (except one, where it describes the UI label)