docs: update release info now that v7 is released

[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)
• [X ] 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
• [ X] Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

https://angular.io/guide/releases
was written at v6, so the release schedule and support dates are now out-of-date

Issue Number: N/A

What is the new behavior?

• Release schedule section lists upcoming at v8 and v9
• Support section lists v4-v7
• Minor edits and table rework for consistency and readability

Note: Technically, v6 is still in Active Support time window until Nov 3, but rather than update this doc again then I went ahead and marked it as LTS. In practice, we don't have any plans I know of to put out a v6+ "active support" release now that v7 is released.

Does this PR introduce a breaking change?

• Yes
• [X ] No

Other information

[IgorMinar]

lgmt with some formatting nitpicks.

[preethamvishy]

I've created a PR for this. Could you check if #26615 is good to go?

[mary-poppins]

You can preview 79cc64c at https://pr26660-79cc64c.ngbuilds.io/.

[mary-poppins]

You can preview 4436532 at https://pr26660-4436532.ngbuilds.io/.

[mary-poppins]

You can preview 5c53e14 at https://pr26660-5c53e14.ngbuilds.io/.

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