refactor(material/datepicker): deprecate unused i18n strings (#25791)

Remove two unused i18n strings from datepicker. Remove `startDateLabel`
and `endDateLabel`.

DEPRECATED:
 * startDateLabel is deprecated because it is not used
 * endDateLabel is deprecated because it is not used

src/material/datepicker/datepicker-intl.ts

@@ -51,10 +51,18 @@ export class MatDatepickerIntl {
   /** A label for the 'switch to year view' button (used by screen readers). */
   switchToMultiYearViewLabel = 'Choose month and year';
 
-  /** A label for the first date of a range of dates (used by screen readers). */
+  /**
+   * A label for the first date of a range of dates (used by screen readers).
+   * @deprecated Provide your own internationalization string.
+   * @breaking-change 17.0.0
+   */
   startDateLabel = 'Start date';
 
-  /** A label for the last date of a range of dates (used by screen readers). */
+  /**
+   * A label for the last date of a range of dates (used by screen readers).
+   * @deprecated Provide your own internationalization string.
+   * @breaking-change 17.0.0
+   */
   endDateLabel = 'End date';
 
   /** Formats a range of years (used for visuals). */

tools/public_api_guard/material/datepicker.md

@@ -471,6 +471,7 @@ export class MatDatepickerIntl {
     calendarLabel: string;
     readonly changes: Subject<void>;
     closeCalendarLabel: string;
+    // @deprecated
     endDateLabel: string;
     formatYearRange(start: string, end: string): string;
     formatYearRangeLabel(start: string, end: string): string;
@@ -481,6 +482,7 @@ export class MatDatepickerIntl {
     prevMonthLabel: string;
     prevMultiYearLabel: string;
     prevYearLabel: string;
+    // @deprecated
     startDateLabel: string;
     switchToMonthViewLabel: string;
     switchToMultiYearViewLabel: string;