Update go-git (to fix failing test)

[tianon]

One of the repositories we test against renamed their upstream branch, which is failing the tests (but cloning it correctly is fixed in newer go-git).

This also means we have to bump our minimum supported Go version, which is unfortunate but fair. If that's a blocker, I'm happy to go deeper and figure out a more minimal go-git version bump, but I don't know if that will actually be satisfiable and bumping seemed pretty reasonable to me.

[tianon]

=== RUN   Example_gitVarnish
--- FAIL: Example_gitVarnish (0.13s)
panic: couldn't find remote ref "refs/heads/master" [recovered]
	panic: couldn't find remote ref "refs/heads/master"

goroutine 1 [running]:
testing.(*InternalExample).processRunResult(0xc0000d9c08, {0x0, 0x0}, 0x2?, 0x0, {0x844da0, 0xc000206450})
	/usr/local/go/src/testing/example.go:96 +0x62e
testing.runExample.func2()
	/usr/local/go/src/testing/run_example.go:59 +0x116
panic({0x844da0?, 0xc000206450?})
	/usr/local/go/src/runtime/panic.go:792 +0x132
github.com/docker-library/bashbrew/pkg/gitfs_test.Example_gitVarnish()
	/wd/pkg/gitfs/tarscrub_test.go:23 +0x311
testing.runExample({{0x89e129, 0x12}, 0x8bec60, {0x8b5055, 0x41}, 0x0})
	/usr/local/go/src/testing/run_example.go:63 +0x2b0
testing.runExamples(0x93dd40?, {0xc0c820, 0x2, 0x4?})
	/usr/local/go/src/testing/example.go:41 +0x125
testing.(*M).Run(0xc0000e15e0)
	/usr/local/go/src/testing/testing.go:2144 +0x71b
main.main()
	_testmain.go:57 +0x9b
FAIL	github.com/docker-library/bashbrew/pkg/gitfs	3.174s

https://github.com/docker-library/bashbrew/actions/runs/17014583269/job/48325202559#step:3:262