doc: revise code review guidelines

Revise the Code Reviews section of the Collaborator Guide to remove
redundant statements, simplify text and structure for easier
comprehension, and remove material that does not reflect current
practices.

PR-URL: #24790
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>

COLLABORATOR_GUIDE.md

@@ -100,28 +100,21 @@ request must pass code review and CI before landing into the codebase.
 ### Code Reviews
 
 At least two Collaborators must approve a pull request before the pull request
-lands. (One Collaborator approval is enough if the pull request has been open
-for more than 7 days.) Approving a pull request indicates that the Collaborator
-accepts responsibility for the change. Approval must be from Collaborators who
-are not authors of the change.
+lands. One Collaborator approval is enough if the pull request has been open
+for more than seven days.
+
+Approving a pull request indicates that the Collaborator accepts responsibility
+for the change.
+
+Approval must be from Collaborators who are not authors of the change.
 
 In some cases, it may be necessary to summon a GitHub team to a pull request for
 review by @-mention.
 See [Who to CC in the issue tracker](#who-to-cc-in-the-issue-tracker).
 
-If you are unsure about the modification and are not prepared to take
-full responsibility for the change, defer to another Collaborator.
-
 If you are the first Collaborator to approve a pull request that has no CI yet,
-please start one (see [testing and CI](#testing-and-ci) for further information
-on how to do that) and post the link to the CI in the PR. Please also start a
-new CI in case the PR creator pushed new code since the last CI run (due to
-e.g., an addressed review comment or a rebase).
-
-In case there are already enough approvals (`LGTM`), a CI run, and the PR is
-open longer than the minimum waiting time without any open comments, please do
-not (only) add another approval. Instead go ahead and land the PR after checking
-the CI outcome.
+please [start one](#testing-and-ci). Post the link to the CI in the PR. Please
+also start a new CI if the PR creator pushed new code since the last CI run.
 
 ### Consensus Seeking