Deprecate titleContent in dialog2 soy template

Description

dialog2.soy

Use of HTML in the title area appears unused (based on cursory scan of `go/sourcegraph`).

It seems that if HTML controls in the header are desired, they should be placed within the "action" section of the header, not as part of the title itself.

Let's deprecate dialog2 `titleContent` in 9.x, remove in 10. If the soy template is documented, direct users to use the action content instead of titleContent.

Taken from https://hello.atlassian.net/wiki/spaces/~mszpyra/pages/631784578/AUI+XSS+audit

Environment

None

QA Kickoff Check-In

Usage described in documentation
Demonstration of all states
Interactive state (screen readers)
Visual regression tests
Unit tests
Navigation (only keyboard / only mouse)
Component render legibly

QA Demo Check-Out

P2 plugin
Browsers (Edge/IE11/Firefox/Chrome/Safari)
Resizing
Zoom in/out
Navigation (Keyboard/Mouse)
Focus
Element status (At Rest/Pressed/Selected/Read Only/Disabled)
Cursors
Tooltips
Audio

Assignee

Unassigned

Reporter

Chris "Daz" Darroch

Labels

None

Changelog entry text

None

Needs Doc

None

Priority

Minor
Configure