[feature] Change instance-stats-randomize to instance-stats-mode with multiple options; implement nodeinfo 2.1

[tsmethurst]

Description

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This pull request removes instance-stats-randomize and instead adds instance-stats-mode, which allows admins to better configure if and how their instance should serve stats.

From the docs:

# String. Allows you to customize if and how stats are served to
# crawlers at the /api/v1|v2/instance and /nodeinfo endpoints.
#
# Note that no matter what you set below, the /api/v1|v2/instance
# endpoints will not be allowed by robots.txt, as these are client
# API endpoints.
#
# "" / empty string (default mode): Serve accurate stats at instance
# and nodeinfo endpoints, and DISALLOW crawlers from crawling
# those endpoints in robots.txt. This mode is equivalent to politely
# asking crawlers not to crawl, but there's no guarantee they will obey,
# as unfortunately many crawlers don't even check robots.txt.
#
# "zero": Serve zeroed-out stats at instance and nodeinfo endpoints,
# and DISALLOW crawlers from crawling those endpoints in robots.txt.
# This mode prevents even ill-behaved crawlers from gathering stats
# about your instance, as all gathered values will be 0. This is the
# safest way of preserving your instance's privacy in terms of stats.
#
# "serve": Serve accurate stats at instance and nodeinfo endpoints,
# and ALLOW crawlers to crawl those endpoints. This mode is useful
# if you want to contribute to fediverse statistics collection projects.
#
# "baffle": Serve randomized, preposterous stats at instance and nodeinfo
# endpoints, and DISALLOW crawlers from crawling those endpoints in robots.txt.
# This mode can be useful to annoy crawlers that don't respect robots.txt.
# Warning that this may draw the ire of crawler implementers who don't
# respect robots.txt, and may therefore put a target on your instance.
#
# Options: ["", "zero", "serve", "baffle"]
# Default: ""
instance-stats-mode: ""

In the process I also implemented the 2.1 version of the nodeinfo schema, which adds links for instance repository and instance homepage :) Because what the hell!

Closes #3723

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.