Promote GetNewBlockV3 and enable by default

[gfukushima]

PR Description

This PR unhides the GetNewBlockV3 and enables it by default.

Fixed Issue(s)

Part of #7779

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[greptile-apps]

PR Summary

The PR promotes the GetNewBlockV3 API and enables it by default.

• CHANGELOG.md: Updated to reflect the promotion of the GetNewBlockV3 API to default.
• teku/src/main/java/tech/pegasys/teku/cli/options/ValidatorOptions.java: Un-hid --block-v3-enabled and set it to default true.
• validator/api/src/main/java/tech/pegasys/teku/validator/api/ValidatorConfig.java: Set DEFAULT_BLOCK_V3_ENABLED to true.

These changes align with the goal of making the Block V3 API the default for block production.

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[tbenr]

Don't know if we really want to expose the --block-v3-enabled.. if we do that we theoretically add it in docs and it will be a problem when we deprecate the option completely (cose it will be a breaking change). I might be wrong, just posing the questions :)

[gfukushima]

Make sense to me, I'll keep it as experimental them.

[rolfyone]

i think leaving it as a dev option thats just enabled by default was what i was thinking, i am hoping that this flag disappears once we prove we don't require it...