feat(conditional-page-builds): make it default

[pieh]

Description

This removes GATSBY_EXPERIMENTAL_PAGE_BUILD_ON_DATA_CHANGES and makes the behaviour our default

[ch25068]

[wardpeet]

Looks good, small nits but nothing blocking

[wardpeet]

shouldn't we move this to the cli? 😬
Can you create a follow up ticket?

[pieh]

Ah, we probably already get things on program for it because we do let yargs know about them (just hide them from --help) -

gatsby/packages/gatsby-cli/src/create-cli.ts

Lines 246 to 261 in 1048eda

	// log-pages and write-to-file were added specifically to experimental GATSBY_EXPERIMENTAL_PAGE_BUILD_ON_DATA_CHANGES feature
	// in gatsby@2. They are useful, but not very applicable (specifically `--write-to-file`) as generic approach, as it only
	// list pages without other artifacts, so it's useful in very narrow scope. Because we don't have alternative right now
	// those toggles are kept for users that rely on them, but we won't promote them and will keep them "hidden".
	.option(`log-pages`, {
	type: `boolean`,
	default: false,
	describe: `Log the pages that changes since last build.`,
	hidden: true,
	})
	.option(`write-to-file`, {
	type: `boolean`,
	default: false,
	describe: `Save the log of changed pages for future comparison.`,
	hidden: true,
	}),

I'll create follow up ticket on that