chore: Remove deprecated src teams commands

[marcleblanc2]

Part of FE-502. #1377 is stacked on this. Stacked on #1375 (base branch is fix/doc-legacy-groups; retarget to main once #1375 merges — the diff shown is only this PR's commit).

Why

Teams were removed in Sourcegraph 7.0. #1257 (shipped in src-cli 7.0.0, 2026-02-25) made src teams refuse to run against 7.x instances (checkTeamsAvailability: "the 'src teams' commands are not available for Sourcegraph versions 7.0 and later ... Teams have been removed"). Six months later the command only works against instances that are no longer supported, yet it still appears in src help and has a reference page on https://sourcegraph.com/docs/cli/references/teams.

While fixing src doc in #1375 I had to special-case teams so the generator wouldn't expand it into 9 new pages for a removed feature. Removing the command is the cleaner answer.

What

• Delete cmd/src/teams*.go and cmd/src/team_members_list.go (9 files, teamsCommands + teamMembersCommands, the team alias, checkTeamsAvailability).

• Drop the teams,team manages teams line from the src usage text in main.go.

• doc_test.go: remove teams.md from expectedDocFiles; legacyGroupsWithoutSubcommandPages is now empty, so every remaining legacy group is expanded into subcommand pages.

• CHANGELOG entry under Unreleased → Removed.

• Remove the Sourcegraph70 feature flag from internal/features/features.go; the teams gate was its only user.

• Also drop extensions,ext and gateway from the src usage text — those commands were removed long ago (chore: Remove deprecated extensions commands #1173 etc.) but were still listed.

Downstream

The generated teams.md disappears from src doc output, so after the next release:

• sourcegraph/sourcegraph#15528 needs "teams.md" removed from OUTPUT_FILES and doc/cli/references/teams.md deleted (I'll update that PR).
• sourcegraph/docs: delete docs/cli/references/teams.mdx (the sync never deletes) — done in docs/cli: Add missing doc pages for src-cli commands docs#1889.

Test plan

$ go build ./... && go vet ./cmd/src/ ./internal/features/
$ go test ./cmd/src/... -count=1
ok
$ go test ./cmd/src -run TestDoc -v
--- PASS: TestDocGeneratesExpectedFiles      (77 files, no teams.md)
--- PASS: TestDocLegacyGroupsHaveSubcommandPages
--- PASS: TestDocRootIndexListsAllCommands
$ go run ./cmd/src teams -h
src: unknown subcommand "teams"
$ go run ./cmd/src team
src: unknown subcommand "team"
$ go run ./cmd/src doc -o /tmp/out && find /tmp/out -name '*team*'
(nothing)

rg -i '\bteams?\b' cmd internal finds no remaining references outside search test fixtures (where "team" is repository content). rg 'Sourcegraph70|extensions|gateway' cmd internal is empty. src (no args) now lists only commands that exist.