fix: use a version of kibana with the fix for fleet-server validations

[mdelapenya]

What does this PR do?

It uses kibana:pr96718, which includes a fix for fleet-server validations.

Why is it important?

We want to make sure the tests are healthy in this point of the history

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have run the Unit tests for the CLI, and they are passing locally
• I have run the End-2-End tests for the suite I'm working on, and they are passing locally
• I have noticed new Go dependencies (run make notice in the proper directory)

Author's Checklist

• Built and push kibana image using the CI (docker.elastic.co/observability-ci/kibana:pr96718

How to test this PR locally

Fleet tests are back to green:

 SUITE="fleet" TAGS="fleet_mode_agent && centos && enroll" TIMEOUT_FACTOR=3 LOG_LEVEL=TRACE BEATS_USE_CI_SNAPSHOT=false DEVELOPER_MODE=true make -C e2e functional-test

Related issues

• Supersedes Fix kibana config for fleet server #1054

[mdelapenya]

The fleet server tests will probably fail at this point: we are working in switching from kibana enrollment to fleet-server enrollment

[elasticmachine]

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Build stats

• Build Cause: Pull request #1055 opened

• Start Time: 2021-04-20T09:10:26.032+0000

• Duration: 40 min 13 sec

• Commit: a483dde

Test stats 🧪

Test	Results
Failed	0
Passed	145
Skipped	0
Total	145

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💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	145
Skipped	0
Total	145