go: bump github.com/google/btree from 1.0.1 to 1.1.2 in /go

• 8e29150 Add coverage of Has in test.
• 3f48535 fix: add Has method for a BTreeG[Item]
• e5eabf3 ci: use github actions matrix
• 770c3d8 Use ~string for Ordered.
• be61f00 Test across many Go versions.
• 1257d40 Create a generic BTreeG for Go 1.18 and beyond.
• ac7cc57 Merge pull request #39 from google/actions
• b5a6bf7 Remove travis badge
• cc35b3d Add verbose flag
• 297f399 Switch to github actions
• See full diff in compare view

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