Clarify required approvals for target promotion and demotion/removal between different target tiers

[jieyouxu]

The promotion and demotion/removal of targets at different Target Tiers is currently too coarse. This PR tries to clarify the degree of consensus/approval required for promotions and demotions of targets between different Target Tiers.

r? @davidtwco (or @wesleywiser)

Rendered

[davidtwco]

I assume we want something along the lines of "PR/r+" for anything related to a Tier 3 target, "MCP/FCP" for anything related to a Tier 2 target, and "RFC/FCP" for anything related to a Tier 1 target

[jieyouxu]

Changed both target promotion and demotion/removal to look like this:

---

Promoting targets

Quick overview for target promotions:

Current target tier	Goal target tier	Proposal	Required approvals
N/A (proposing a new target)	3	PR	r+ (compiler leads)
3	2	MCP	FCP
2	1	RFC	FCP

• Proposing a new target
  • Propose using: PR
  • Approve using: r+ (compiler leads)
  • You can r? compiler_leads on the PR to roll one of the compiler leads as the reviewer.
  • Open a PR with the new target (w/ relevant documentation updates) and document adherence to the
    [target tier policy][tier_policy] in the description.
  • New targets must start as Tier 3.
  • New targets should be assigned to the compiler team co-leads to check for any licensing concerns
    and to ensure that any demands on the project infrastructure are considered and checked with
    relevant teams.
• Promoting a target from Tier 3 to Tier 2
  • Propose using: MCP
  • Approve using: FCP
  • Open a MCP with the target and document adherence to the [Tier 2 target policy][tier_policy] in
    the description.
• Promoting a target from Tier 2 to Tier 1
  • Propose using: RFC
  • Approve using: FCP
  • Open a RFC with the target and document adherence to the [Tier 1 target policy][tier_policy] in
    the RFC text.

Demoting or removing targets

Quick overview for target demotions and removals:

Current target tier	Goal target tier	Proposal	Required approvals
1	2 (or removal)	RFC	FCP
2	3 (or removal)	MCP	FCP
3	N/A (removing a tier 3 target)	PR	r+ (compiler leads)

• Demoting a target from Tier 1 to Tier 2, or removing a Tier 1 target
  • Propose using: RFC
  • Approve using: FCP
  • Open a RFC with the target and rationale.
  • Example: RFC: Demote i686-pc-windows-gnu to Tier 2
    #3771
• Demoting a target from Tier 2 to Tier 3, or removing a Tier 2 target
  • Propose using: MCP
  • Approve using: FCP
  • Open a MCP with the target and rationale.
• Removing a Tier 3 target
  • Propose using: PR
  • Approve using: r+ (compiler leads)
  • Open a PR with the target and rationale for removing the Tier 3 target.

[jieyouxu]

@rustbot review