Update dependabot to pull submodule tags instead of latest main

[glopesdev]

Maybe this is different for aeon_mecha, but on aeon_acquisition we actually don't use anything in production which is not tagged and published on NuGet and we wouldn't want to give the false impression that some API is available because it has been merged to main.

As an example on aeon_acquisition the latest release tag would be commit: SainsburyWellcomeCentre/aeon_acquisition@63a4c0e

Originally posted by @glopesdev in #69 (comment)

[lochhh]

Not sure we can easily do this (see dependabot/dependabot-core#1639), but we can consider reducing the update frequency. As these dependabot PRs are not automatically merged, we can always close the PRs if the update is unwanted.

Maybe:
https://docs.renovatebot.com/modules/manager/git-submodules/#updating-to-specific-tag-values