Cherry-pick #15937 to 7.x: Upgrade go-ucfg to v0.8.1 release

[blakerouse]

Cherry-pick of PR #15937 to 7.x branch. Original message:

What does this PR do?

Updates the vendored go-ucfg to v0.8.1.

Note: This includes a breaking change that removes the variable substition from environment variables when they are read from the configuration. elastic/go-ucfg#139

Why is it important?

Gets the latest fixes and additions including:

Added

• Add support for HJSON. HJSON seems to not be implemented go-ucfg#131
• Add new parse.Config to adjust parsing of varibles returned by a Resolve. Fix parsing of environment variables to prevent object parsing. #132 go-ucfg#139
• Add call to InitDefaults when map, primitives, or structs implement Initializer interface during Unpack. Initialize defaults in sub-structures go-ucfg#104

Changed

• Moved internal/parse to parse module. Fix parsing of environment variables to prevent object parsing. #132 go-ucfg#139
• Add parse.Config to resolvers return. Fix parsing of environment variables to prevent object parsing. #132 go-ucfg#139

Fixed

• Prevent Validate from being called when value is a pointer or interface and is nil. Validate being called on structs that are nil go-ucfg#144
• Call Validate on custom slice types. The Validate method is not always called go-ucfg#133
• Call Validate on custom map types. More validator tests go-ucfg#136
• Disabled object parsing of environment variables. Fix parsing of environment variables to prevent object parsing. #132 go-ucfg#139
• Apply validation to defaults passed into Unpack when Config doesn't contain a value. Apply validators to default values go-ucfg#42

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

Author's Checklist

• Does not break beats reading and parsing configuration files

How to test this PR locally

Related issues

Use cases

Screenshots

Logs

[blakerouse]

@urso Recommended to do them together, but I can easily split the apart if you prefer.

[ycombinator]

@blakerouse Yeah, I prefer keeping the backport PRs 1-1 with the original ones as it makes it quite easy to compare the changes between the backport and original PRs, at least for me. With combined PRs, especially with a high number of LOC, I'm more likely to miss something.

[blakerouse]

@ycombinator Works for me, I removed the cherry-pick of 0.8.2, will do that after this lands in 7.x.

[ycombinator]

LGTM.