Set eks_endpoint_access to immutable on upgrade to allow merge of #3004

[asmacdo]

Context

As discussed in the community meeting there is concern that merging #3004 will break existing deployments.

On upgrade this can be worked around by:

1. setting the value to the previous default

https://github.com/nebari-dev/nebari/pull/3004/files#diff-85a0aac851ba9454fb93be1ed93f3907ecbcea446f599a91424183dbce3cec75R169-L171

    eks_endpoint_access: Optional[
        Literal["private", "public", "public_and_private"]
    ] = "public"

2. making this value immutable. example

@viniciusdc could you please sanity check?

Value and/or benefit

Allow merge of 3004, which improves security

Anything else?

No response

[viniciusdc]

@dcmcand once you are back, can you have a look at this discussion?