Remove CodeRabbit references (#2960)

CodeRabbit is being removed from every org. This drops the remaining references to it in this repo: CodeRabbit config files, their ignore and REUSE entries, the PR-size workflow that asks @coderabbitai to assess merged PRs, and the CodeRabbit example in the PR template.

Author: JuaniRios

.coderabbit.yaml

@@ -14,7 +14,7 @@ Never include:
 - A restated diff, or a list of changed files.
 - The story of the investigation, or of the review rounds.
 - Test counts, or lists of lint, fmt, CI, or review runs. The checks tab shows them.
-- A tool-generated summary (for example "Summary by CodeRabbit").
+- A tool-generated summary.
 
 Writing:
 - Plain, simple English. One idea per sentence. Short sentences. Active voice with a real subject.

.github/pull_request_template.md

@@ -41,7 +41,6 @@ path = [
   "meta/**/",
   "test-resources/**/",
   "audit/**/",
-  ".coderabbit.yaml",
 ]
 SPDX-FileCopyrightText = "Copyright (c) 2020 Rain Open Source Software Ltd"
 SPDX-License-Identifier = "LicenseRef-DCL-1.0"