fix(linode): don't try to create existing records

[wbh1]

Description

A bug in the Linode provider can result in an unbounded explosion of TXT records being created. For example, if external-dns attempts to create a CNAME in a zone that has a conflicting A record, it will fail but will still create 2 new TXT records every single time.

Instead, we can just skip creating records that already exist since they should only ever be in the list of updates.

Fixes #3045

Checklist

• Unit tests updated
• End user documentation updated

[k8s-ci-robot]

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[mloiseleur]

Hello @wbh1,

This PR also needs a test, in order to ensure it won't happen again.

[wbh1]

Thanks, @mloiseleur. Test updated 😄

[mloiseleur]

/ok-to-test
/lgtm

[Raffo]

/approve

[k8s-ci-robot]

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This pull-request has been approved by: Raffo

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