📖 Update CAPI support and guarantees for v1.8

[chandankumar4]

What this PR does / why we need it:

This PR updates the supported versions for the next release cycle (1.8).

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes Task: https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/release/release-tasks.md#update-supported-versions

@kubernetes-sigs/cluster-api-release-team

/area documentation

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[sbueringer]

This has overlap with @fabriziopandini's PR (#10531)

[sbueringer]

Okay I think it's easier to rebase if we merge this one first

/ok-to-test
/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 11a2659a3b276daf381a744b4dc64e0dde07c30e

[k8s-ci-robot]

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This pull-request has been approved by: sbueringer

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[fabriziopandini]

@sbueringer ok to rebase my PR, but we should find a way to set EOL for 1.5 in the current version of the book without cherry-picking the new 1.8 columns that should exist only on main

[sbueringer]

Let's open a manual cherry-pick once your PR merges

[fabriziopandini]

The two PR are orthogonal now
@chandankumar4 could you cherry-pick your PR to 1.7 (without the 1.8 column)?

[sbueringer]

@fabriziopandini Which part of this PR? Dropping the v1.4 column in the versions table? If I see correctly everything else shouldn't be cherry-picked

[fabriziopandini]

Yes, I think it makes sense to backport everything in this PR except for the 1.8 column

[sbueringer]

I'll open the PR (just to avoid confusion)