docs: add final release date and update future schedule

[jenniferfell]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

https://angular.io/guide/releases#release-schedule
contains versions 8 and 9

https://angular.io/guide/releases#support-policy-and-schedule
has the version 8 release date as May 22

Issue Number: N/A

What is the new behavior?

https://angular.io/guide/releases#release-schedule
contains versions 9 and 10, using the same months we previously published for estimates

https://angular.io/guide/releases#support-policy-and-schedule
has the version 8 release date as May 28

Does this PR introduce a breaking change?

• Yes
• No

Other information

[jenniferfell]

@IgorMinar and @manughub : Please confirm the future estimated release dates and the final release date for 8.0.0 (for support purposes). Thanks.

[mary-poppins]

You can preview e7fb015 at https://pr30712-e7fb015.ngbuilds.io/.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}