Remove old cache directories

[fd0]

This PR adds code which warns the user when old cache directories are present. A cache directory is old if the modification timestamp is older than 30 days. Users can then use the new cache command with --cleanup, which will remove all old directories.

When a cache directory is used, the modification timestamp is set to the current time and date so that it isn't in the list of old directories.

Closes #1318

[codecov-io]

Codecov Report

Merging #1436 into master will decrease coverage by 0.2%.
The diff coverage is 18.84%.

@@            Coverage Diff             @@
##           master    #1436      +/-   ##
==========================================
- Coverage   51.12%   50.92%   -0.21%     
==========================================
  Files         136      136              
  Lines       13612    13735     +123     
==========================================
+ Hits         6959     6994      +35     
- Misses       5800     5884      +84     
- Partials      853      857       +4

Impacted Files	Coverage Δ
internal/fs/file.go	0% <0%> (ø)	⬆️
internal/cache/cache.go	25.22% <13.51%> (-4.65%)	⬇️
cmd/restic/global.go	21.09% <26.08%> (-0.64%)	⬇️
internal/cache/dir.go	6.66% <28.57%> (+6.66%)	⬆️
cmd/restic/lock.go	52.5% <0%> (-5%)	⬇️
cmd/restic/exclude.go	27.71% <0%> (-3.02%)	⬇️
internal/backend/s3/config.go	74.41% <0%> (-2.86%)	⬇️
cmd/restic/cleanup.go	34.78% <0%> (-2.72%)	⬇️
internal/backend/b2/b2.go	77.36% <0%> (-2.04%)	⬇️
cmd/restic/cmd_backup.go	26.72% <0%> (-0.87%)	⬇️
... and 6 more

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