[2.0.0-dev] upgrade reproducible build image to Debian11; effectively making Ubuntu20 minimum runtime environment

[spoonincode]

The reproducible build image is currently based on Debian10 meaning any builds there should work on any distro with glibc 2.28+ -- that includes Ubuntu18 that we don't officially support in the 1.x range.

Bump the build image to Debian11 increasing requirement to glibc 2.31+ which drops the ability to run on Ubuntu18 but still will run on Ubuntu20. It will also drop EL8.

I think we are a little late on this, as other similar projects made the move a while ago, e.g. https://github.com/bitcoin/bitcoin/pull/29987

We have a little kludge here

spring/tools/tweak-deb.sh

Lines 4 to 12 in 2f236ca

	# Tweaks a couple aspects of the built .deb's control file:
	# 1. Removes Installed-Size field; this isn't reproducible for some reason, possibly different filesystems
	# reporting different sizes for directories?
	# 2. Removes all but the first libc Depends rule as the rest are unnecessarily restrictive. The original was being
	# generated as,
	# libc6 (>= 2.27), libc6 (>> 2.28), libc6 (<< 2.29), libcurl4 (>= 7.16.2), libgcc1 (>= 1:3.3), libgmp10, zlib1g (>= 1:1.2.0)
	# and the included sed rule within this script will reduce it to
	# libc6 (>= 2.27), libcurl4 (>= 7.16.2), libgcc1 (>= 1:3.3), libgmp10, zlib1g (>= 1:1.2.0)
	# This may need to be tweaked in the future further; clearly not ideal.

fwiw I did check the resulting .deb file looks correct still after this upgrade:

Depends: libc6 (>= 2.30), libcurl4 (>= 7.16.2), libgcc-s1 (>= 3.3), libgmp10, zlib1g (>= 1:1.2.0)