kvserver/rangefeed: metamorphically enable RangefeedUseBufferedSender

[wenyihu6]

kvserver/rangefeed: rename .buffered_stream_sender. to .buffered_sender.

This patch renames kv.rangefeed.buffered_stream_sender.enabled to
kv.rangefeed.buffered_sender.enabled to align with variable and
struct names better.

Epic: none
Release: none

---

kvserver/rangefeed: metamorphically enable RangefeedUseBufferedSender

This patch metamorphically enables the cluster setting
RangefeedUseBufferedSender.

Release note: none
Part of: #135332

[blathers-crl]

Your pull request contains more than 1000 changes. It is strongly encouraged to split big PRs into smaller chunks.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[wenyihu6]

How do we feel about lifting the restriction here

cockroach/pkg/kv/kvserver/replica_rangefeed.go

Lines 102 to 107 in a907659

	settings.WithValidateBool(func(_ *settings.Values, b bool) error {
	if buildutil.CrdbTestBuild || !b {
	return nil
	}
	return ErrBufferedSenderNotSupported
	}),

?

Update: seems fine to leave it for now given roachtests do not use metamorphic constants https://cockroachlabs.slack.com/archives/C023S0V4YEB/p1733846019532919.

[stevendanna]

Thanks!

[wenyihu6]

TFTR!

bors r=stevendanna

[wenyihu6]

Extended CI failure TestLogGrowthWhenRefreshingPendingCommands looks unrelated.

[craig]

Build succeeded:

• linux_amd64_build
• windows_build
• local_roachtest
• linux_arm64_build
• examples_orms
• macos_amd64_build
• acceptance
• linux_amd64_fips_build
• local_roachtest_fips
• docker_image_amd64
• unit_tests
• check_generated_code
• macos_arm64_build
• lint