fix: [Analytics:Graph page]Dialog modal missing title from announcement (elastic#217827)

Closes: elastic#214760

**Description**
Dialog modal, flyout, field visible title should be announced for the
users, especially using assistive technology to know what dialog modal,
flyout opened, what field is active and what is needed to enter in it.

**Changes made:**

1. Set correct value for` aria-labelledby` attr.

Author: alexwizp

src/core/packages/overlays/browser/src/modal.ts

@@ -7,7 +7,7 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-import type { EuiConfirmModalProps } from '@elastic/eui';
+import type { EuiConfirmModalProps, EuiModalProps } from '@elastic/eui';
 import type { MountPoint, OverlayRef } from '@kbn/core-mount-utils-browser';
 
 /**
@@ -22,6 +22,7 @@ export interface OverlayModalConfirmOptions {
   'data-test-subj'?: string;
   defaultFocusedButton?: EuiConfirmModalProps['defaultFocusedButton'];
   buttonColor?: EuiConfirmModalProps['buttonColor'];
+  'aria-labelledby'?: EuiConfirmModalProps['aria-labelledby'];
   /**
    * Sets the max-width of the modal.
    * Set to `true` to use the default (`euiBreakpoints 'm'`),
@@ -66,4 +67,5 @@ export interface OverlayModalOpenOptions {
   closeButtonAriaLabel?: string;
   'data-test-subj'?: string;
   maxWidth?: boolean | number | string;
+  'aria-labelledby'?: EuiModalProps['aria-labelledby'];
 }

x-pack/platform/plugins/private/graph/public/components/source_modal.tsx

@@ -16,7 +16,7 @@ export function SourceModal(props: SourcePickerProps) {
   return (
     <div css={sourceModalStyles}>
       <EuiModalHeader>
-        <EuiModalHeaderTitle>
+        <EuiModalHeaderTitle id="source-modal-title">
           <FormattedMessage
             id="xpack.graph.sourceModal.title"
             defaultMessage="Select a data source"

x-pack/platform/plugins/private/graph/public/services/source_modal.tsx

@@ -32,6 +32,9 @@ export function openSourceModal(
         onSelected(indexPattern);
         modalRef.close();
       }}
-    />
+    />,
+    {
+      'aria-labelledby': 'source-modal-title',
+    }
   );
 }