refactor(terraform): aws provider upgrade to v6

[katapultcloud]

Description

This PR updates Terraform provider lock data to include AWS provider v6 and aligns template sync ignore behavior for addon-specific local files.

Compared to main, this branch:

• Upgrades locked hashicorp/aws provider version in .terraform.lock.hcl from 5.100.0 to 6.42.0 (including checksum updates).
• Normalizes the locked hashicorp/helm constraint in .terraform.lock.hcl to >= 3.0.0.
• Adds migrations.tf and .templatesyncignore.local to .templatesyncignore.
• Adds .templatesyncignore.local to prevent overriding addon-specific templated files.
• Syncs helm_keyring description wording in variables-addon.tf and generated README.md.

No issue is fixed by this PR.

Type of change

• A bug fix (PR prefix fix)
• A new feature (PR prefix feat)
• A code change that neither fixes a bug nor adds a feature (PR prefix refactor)
• Adding missing tests or correcting existing tests (PR prefix test)
• Changes that do not affect the meaning of the code like white-spaces, formatting, missing semi-colons, etc. (PR prefix style)
• Changes to our CI configuration files and scripts (PR prefix ci)
• Documentation only changes (PR prefix docs)

How Has This Been Tested?

• Plan and apply with the updated lock file to ensure no issues with provider versions.

[jaygridley]

LGTM