Enable backups in the crypto crate by default

[poljar]

This PR enables backups in the crypto crate by default and removes the
backups_v1 feature, for better or worse.

• Public API changes documented in changelogs (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (beb01ea) 82.03% compared to head (a5c4098) 82.36%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2813      +/-   ##
==========================================
+ Coverage   82.03%   82.36%   +0.32%     
==========================================
  Files         204      208       +4     
  Lines       21024    21391     +367     
==========================================
+ Hits        17248    17618     +370     
+ Misses       3776     3773       -3     

Files	Coverage Δ
crates/matrix-sdk-crypto/src/backups/mod.rs	72.92% <ø> (ø)
crates/matrix-sdk-crypto/src/gossiping/machine.rs	77.80% <ø> (+6.62%)	⬆️
crates/matrix-sdk-crypto/src/identities/device.rs	79.68% <ø> (-1.62%)	⬇️
crates/matrix-sdk-crypto/src/lib.rs	100.00% <ø> (ø)
crates/matrix-sdk-crypto/src/machine.rs	86.69% <ø> (+2.20%)	⬆️
crates/matrix-sdk-crypto/src/olm/account.rs	88.66% <ø> (+1.00%)	⬆️
...atrix-sdk-crypto/src/olm/group_sessions/inbound.rs	83.83% <ø> (+1.40%)	⬆️
crates/matrix-sdk-crypto/src/olm/utility.rs	85.00% <ø> (-4.48%)	⬇️
crates/matrix-sdk-crypto/src/store/mod.rs	87.37% <100.00%> (+1.17%)	⬆️
...atrix-sdk-crypto/src/types/cross_signing/master.rs	83.87% <ø> (ø)
... and 1 more

... and 13 files with indirect coverage changes

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[bnjbvr]

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