bump ubuntu

Merge pull request #286 from rust-embedded/r046

release 0.4.6

Merge pull request #283 from rust-embedded/r045

release 0.4.5

Merge pull request #278 from rust-embedded/r044

release 0.4.4

Merge pull request #274 from rust-embedded/derive-reg-cluster

shorthand derive across cluster

Merge pull request #272 from rust-embedded/fix-mwv

Fix optional fspec in set_field_modified_write_values

Merge pull request #271 from rust-embedded/r041

release 0.4.1

Merge pull request #264 from rust-embedded/merge

Keep position when merge fields, release 0.4

Merge pull request #260 from rust-embedded/r0321

release 0.3.21

Merge pull request #254 from rust-embedded/r0320

release 0.3.20