Proposal: Rename `extendFootprintTtl.extendTo` to `minimumTtl` for clarity

[Shaptic]

There was a lengthy discussion on Discord about how the naming of extendTo seems to imply an absolute ledger sequence as the parameter (i.e. extend the lifetime to ledger N). In actuality, this sets the entries' time to live to at least N more ledgers (i.e. extend, but more accurately set the TTL to ledger N).

This PR proposes a breaking change to make it minimumTtl to clarify this.

[github-actions[bot]]

Size Change: +229 B (0%)

Total Size: 3.21 MB

Filename	Size	Change
dist/stellar-base.js	2.36 MB	+221 B (0%)
dist/stellar-base.min.js	855 kB	+8 B (0%)

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