feat(http server)!: remove deprecated encoding option

[pront]

Summary

Motivation

The HTTP server encoding option has been deprecated since Vector 0.50.0. Its
migration window has elapsed, and retaining its compatibility decoder keeps an
obsolete configuration path alive.

Changes

• Remove encoding from the http and http_server source configurations.
• Reject configurations that still use the removed field.
• Regenerate component documentation and record the removal in the deprecations index.
• Add a breaking-change migration guide mapping each legacy encoding to decoding and framing.

Vector configuration

sources:
  in:
    type: http_server
    address: 0.0.0.0:8080
    decoding:
      codec: text
    framing:
      method: newline_delimited

How did you test this PR?

• cargo test --lib --no-default-features --features sources-http_server http_server::tests::rejects_removed_encoding_field
• make VDEV=target/debug/vdev check-clippy
• target/debug/vdev check changelog-fragments
• target/debug/vdev deprecation check
• cargo fmt --check
• vector validate rejects a configuration containing encoding: text.

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. A breaking changelog fragment is included.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

The removal is recorded as enacted in Vector 0.58.0.