Backports for 1.12

[KristofferC]

Backported PRs:

• Update registries during dev #4186
• Add a pkg message if julia_version is being specified #4331
• Clear the registry cache on Pkg module end #4393
• Adapt delayed delete mechanism #4392
• Make julia compat checking respect julia_version #4403

Need manual backport:

• Fix deleting in compat mode #4402

Contains multiple commits, manual intervention needed:

• Fix completion at the end of a complete line #4373

Non-merged PRs with backport label:

• STDLIBS_BY_VERSION: Check sorted & require same minor #4414 (cherry picked from commit 0e123a2)