feat(migrations): Add migration for inputs.tail from_beginning option

[skartikey]

Summary

The from_beginning option in inputs.tail was deprecated in v1.34.0 for removal in v1.40.0, replaced by initial_read_offset. This adds a migration that maps from_beginning = true to initial_read_offset = "beginning" and false to "saved-or-end" on telegraf config migrate. When initial_read_offset is already set it takes precedence and is kept, dropping from_beginning; if the two disagree the migration warns. The option removal ships separately (#19132) so users can migrate while still on v1.39.x.

Checklist

• No AI generated code was used in this PR

Related issues

related to #19088

[srebhan]

Thanks @skartikey! Can we please split the PR into adding the migration and another PR for removing the option. This way we can merge the migration part now and remove the option shortly before v1.40.0 i.e. after the last bug-fix release for v1.39.x... Same for the other PRs...

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_arm64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

☺️ This pull request doesn't significantly change the Telegraf binary size (less than 1%)

📦 Click here to get additional PR build artifacts

Artifact URLs

. DEB	. RPM	. TAR . GZ	. ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz

[srebhan]

Perfect, thanks!